Russian President Vladimir Putin has threatened Ukraine with further action after a missile and drone barrage hit Kyiv and other big cities.
The attacks prompted an international outcry, with Liz Truss and fellow leaders of G7 nations expected to hold crisis talks on Tuesday with Ukraine’s leader Volodymyr Zelensky.
UK ministers described the attacks as “vile” and “unacceptable” as the death toll mounted in civilian areas.
The strikes on Ukrainian cities were in response to an attack on a bridge linking Russia and Crimea.
Ukraine’s capital Kyiv was targeted for the first time in months, while there were also reports of explosions in Dnipro, Lviv, Ternopil, Khmelnytskyi, Zhytomyr and Kropyvnytskyi.
Mr Putin confirmed they were a retaliation for Ukraine’s strike on the Kerch Bridge, a crossing between Russia and annexed Crimea that has strategic and symbolic importance.
He said if Ukraine continues to mount “terrorist attacks” on Russia, which invaded its neighbour in February, the Kremlin’s response will be “tough and proportionate to the level of threats”.
The attack on Kyiv resulted in explosions in the city’s Shevchenko district, a central area which includes the historic old town as well as several government offices.
BBC journalist Hugo Bachega was broadcasting live when the attack happened, with the sound of a missile overhead and subsequent explosion captured on his microphone.
He could be seen ducking for cover as the blast rocked the capital during his piece on BBC World. He got to safety with his crew.
Lesia Vasylenko, a member of Ukraine’s parliament, posted a photograph on Twitter showing at least one explosion happened near the main building of the Kyiv National University in the capital’s centre.
“What is Russia trying to hit?” she asked. “The national university? The park? Or the playground?”
A glass pedestrian bridge, a popular attraction, was also struck.
“People would have been jogging this morning,” Ms Vasylenko said.
At least 11 people were killed and 64 injured in the attacks across Ukraine, according to the emergency services.
Foreign Secretary James Cleverly offered the UK’s “ongoing moral and practical support” to his counterpart in Kyiv, Dmytro Kuleba.
“Russia’s firing of missiles into civilian areas of Ukraine is unacceptable,” he said.
“This is a demonstration of weakness by Putin, not strength.”
Security minister Tom Tugendhat added: “Targeting civilians is a vile act.
“Russian war crimes start as a record of failure and disgrace.”
The UK’s ambassador to Ukraine Melinda Simmons said British staff in Kyiv were safe and said her thoughts are with those hurt in the city.
Mr Zelensky said: “The world once again saw the true face of a terrorist state that is killing our people.”
Following talks with Ms Truss he said Ukraine counts on the UK’s “leadership in consolidating international political and defence support for Ukraine, in particular regarding the protection of our skies” and also the “further isolation of Russia”.
Downing Street said Ms Truss viewed the “appalling attacks” as “a sign of Ukrainian success and increasing desperation by Putin in response”.
The G7 call will take place at 1pm on Tuesday, with Mr Zelensky joining for part of the discussion.
