Ukrainian officials have warned that a new Russian offensive has begun in the country's east, as the United States continued to emphasise it would not be sending soldiers to fight a war against Moscow.
President Volodymyr Zelenskyy's chief of staff, Andriy Yermak, warned "the second phase of the war has started", amid Russian bombardments against Ukrainian cities.
Russian forces took control of the city of Kreminna in eastern Ukraine on Tuesday and Ukrainian troops have withdrawn from the city, the regional governor told a briefing.
"Kreminna is under the control of the [Russians]. They have entered the city," said Serhiy Gaidai, the governor of the Luhansk region.
The United Nations said the war's civilian death toll had surpassed 2,000 since Russia invaded on February 24, reaching 2,072 as of midnight on April 17.
Serbia's pro-Russian government has accused Ukrainian spies of calling in bomb threats against flights into Russia.
Victims of Serbian war crimes in Bosnia, meanwhile, warn Ukrainians that there is a long road to justice.
Ukrainian officials say Russia's new offensive has begun
President Zelenskyy said Russian troops had "begun the battle for the Donbas, for which they have been preparing for a long time".
"[A] significant part of the entire Russian army is now concentrated on this offensive," he added.
Local media reported a series of explosions, some powerful, along the front line in the Donetsk region, with shelling taking place in Marinka, Slavyansk and Kramatorsk.
Ukrainian local officials and local media also said explosions were heard in Kharkiv in the north-east of Ukraine, Mykolaiv in the south and Zaporizhzhia in the south-east.
In the besieged southern city of Mariupol, Ukrainian fighters are reportedly holding out in underground shelters beneath the vast Azovstal steel plant, along with about 1,000 civilians.
Mr Zelenskyy and Mr Yermak echoed comments by Ukraine's top security official Oleksiy Danilov, who earlier said that Russia had launched its new offensive on Monday morning, local time.
Russian forces had attempted to break through Ukrainian defences "along almost the entire front line of Donetsk, Luhansk and Kharkiv regions," Mr Danilov said.
Mr Yermak wrote on the Telegram messing app that Ukrainians should be confident the country's forces could hold off the offensive.
"Believe in our army. It is very strong," he said.
Russia has intensified attacks on several Ukrainian cities, but says it is concentrating its efforts on securing full control over eastern Ukraine's Donbas region — made up of the Donetsk and Luhansk regions.
Russia's Defence Ministry said it had hit hundreds of military targets in Ukraine overnight.
Meanwhile, Donetsk regional governor Pavlo Kirilenko said on his Telegram channel said that Russian shelling had killed four people there.
Donbas has been the focal point of Russia's campaign to destabilise Ukraine, starting in 2014 when the Kremlin used proxies to set up two separatist "people's republics" in the former Soviet state.
It is also home to much of Ukraine's industrial wealth, including coal and steel.
Russia also bombarded the western city of Lviv and numerous other targets across Ukraine in what appeared to be an intensified bid to grind down the country's defences ahead of the eastern offensive.
Ukraine said a Russian missile attack killed seven people in Lviv — the first civilian victims in the city, which lies 60 kilometres from the Polish border.
Lviv had become a haven for civilians fleeing the fighting elsewhere, as well as a major gateway for NATO-supplied weapons to Ukraine.
The Russian attack hit three military infrastructure facilities and an auto shop, according to Lviv regional governor Maksym Kozytskyy.
He said the wounded included a child.
In Kharkiv, a man and a woman were killed when shells hit a playground near a residential building, authorities said.
Evacuation issues continue
Ukraine was unable, for a third successive day, to secure Russia's agreement to establish any humanitarian corridors to evacuate civilians trapped in cities and towns, Deputy Prime Minister Iryna Vereshchuk said.
Efforts to create safe passage for civilians to leave Mariupol have failed repeatedly, with each side blaming the other. Russia has denied targeting civilians.
"According to Mariupol: (the) Russians refuse to provide a corridor for the exit of civilians in the direction of Berdyansk," Ms Vereshchuk said.
She said "difficult negotiations" were taking place to try to arrange humanitarian corridors in the southern region of Kherson and in Kharkiv.
The US is 'not going to fight a war with Russia'
The White House has emphasised it does not intend to deploy US troops to Ukraine to fight against the Russian invasion, although it is weighing additional sanctions against Moscow.
White House Press Secretary Jen Psaki said there were no plans for US President Joe Biden to visit Kyiv, nor plans to provide military support beyond the "historic" supply of arms to Ukraine she said had "helped them effectively fight this war".
Sanctions against Russia — in tandem with ongoing military and economic assistance to Ukraine — was "also strengthening [Ukraine's] hand at the negotiating table", she said.
"You will see us continue to expand our sanctions targets and continue to take steps to both further tighten our sanctions to prevent evasion and put in place additional sanctions."
Senator Chris Coons of Mr Biden's Democratic party has argued that the President and US Congress need to consider having American boots on the ground in Ukraine.
Ms Psaki said that, while Senator Coons was a close friend of Mr Biden, the administration "respectfully disagree[d]" with his views on Ukraine.
"The President has no plans to send troops to fight a war with Russia," she said.
"He doesn't think that's in our national security interests — in the interests of the American people."
Kramatorsk school damaged by shelling
A school building was damaged in an attack in the city of Kramatorsk, the blast shattering its windows.
"We came to work and saw that everything had been destroyed," said Viktor Herosimenko, who has been a guard at the school for the past 12 years.
"What will be next, nobody knows. Well, a lot of glass was broken, but everything will be fine … we will get over all this. We only must have a clear and peaceful sky," the 70-year-old said.
The school had been spared from shelling until now.
Russia says it only bombs military targets and denies attacking civilians.
According to local authorities, eight residential buildings, the school and a facility were damaged in the early morning attack on the city in eastern Ukraine.
There were no reports of deaths or injuries.
Kramatorsk — where earlier this month almost 60 people died when a rocket hit a crowded railway station — has been the target of shelling in the past few days.
Ukraine rejects bomb threat accusation from Serbia
Serbian President Aleksandar Vučić has claimed that the foreign intelligence services of Ukraine and an unidentified European Union nation are responsible for bomb threats against Air Serbia flights to Russia.
The pro-Russian Serbian leader did not provide evidence for his claim.
Other Serbian officials alleged that the threats were being sent from Ukraine or Poland.
Ukraine's Foreign Ministry spokesman, Oleg Nilolenko, called the allegations false.
The Serbian national carrier is the only European airline, besides Turkish air companies, that has not joined EU flight sanctions against Russia over its war in Ukraine.
Mr Vučić was re-elected earlier this month, having refused to join Western sanctions against Russia despite formally seeking membership in the European Union for Serbia.
Russian President Vladimir Putin congratulated Mr Vučić, saying that the outcome confirmed "broad public support" for his independent foreign policies.
The Russian leader voiced hope that Mr Vučić's activities would help further strengthen the "strategic partnership" between Russia and Serbia.
Last week, right-wing groups rallied in the Serbian capital of Belgrade to protest the government's support of suspending Russia from the UN Human Rights Council.
Bosnians warn of long road to justice
Survivors of war crimes committed during Bosnia's war 30 years ago say the victims of human rights abuses in Ukraine can learn from their experience, which was lengthy and painful.
It took decades to arrest and try the wartime Bosnian Serb leaders, and more than 7,000 people still remain unaccounted-for.
But the UN war crimes tribunal for the former Yugoslavia eventually convicted 83 high-ranking political and military officials and transferred a mountain of evidence against lower-ranking suspects to their home countries for prosecution.
The guilty were collectively sentenced to more than 700 years in prison.
Munira Subasic helped create Mothers of Srebrenica to demand that bodies be identified and those responsible brought to justice.
To date, almost 90 per cent of those reported missing from the fall of Srebrenica have been accounted-for.
"Russia's denials of massacres its soldiers are now obviously committing in Ukraine sound to me the same as Srebrenica genocide denial," Ms Subasic said.
"But, if survivors are persistent, the truth will prevail."
ABC/wires
