Syrian government forces were locked in heavy fighting around the central city of Hama on Thursday, trying to halt an advance of Islamist-led rebels, a war monitor said.
The fighting around Hama follows a rapid offensive by the rebels, who in just days captured large chunks of territory, including Syria's second city Aleppo, from President Bashar al-Assad's control.
Strategically located in central Syria, Hama is crucial for the army's efforts to protect the capital, Damascus.
By late Wednesday, the Syrian Observatory for Human Rights monitor said rebel fighters had "surrounded Hama city from three sides".
"Violent clashes took place during the night between the rebels and the regime forces", particularly in the Jabal Zayn al-Abidin area, just north of Hama, said the Britain-based monitor.
The head of the Observatory, Rami Abdel Rahman, said government troops were engaged in "fierce resistance and trying to stop the rebels' advance".
Syrian state media quoted a military source late Wednesday as saying Russian and Syrian air forces, alongside artillery units, had conducted "concentrated strikes on the... terrorists" in the Hama area.
Maya, a 22-year-old student who gave her first name only for security concerns, said she and her family were staying at home as the fighting rages outside.
"We have been hearing non-stop the sounds of explosions and shelling," she told AFP by telephone from Hama.
"We don't know what's going on outside."
The Observatory, which relies on a network of sources in Syria, says 704 people, mostly combatants but also 110 civilians, have been killed in Syria since the violence erupted last week.
It marks the most intense fighting since 2020 in a country already ravaged by civil war, which erupted with the repression of pro-democracy protests in 2011.
Key to the rebels' successes since the start of the offensive last week was the takeover of Aleppo, which in more than a decade of war had never entirely fallen out of government hands.
The head of the Islamist rebel group Hayat Tahrir al-Sham (HTS), Abu Mohammad al-Jolani, on Wednesday visited Aleppo's landmark citadel.
Jolani was seen waving to supporters from an open-top car as he visited the historic fortress, in images posted on the rebels' Telegram channel.
While the advancing rebels found little resistance earlier in their offensive, the fighting around Hama has been especially fierce.
Assad ordered a 50-percent raise in career soldiers' pay, state news agency SANA reported, as he seeks to bolster his forces for the counteroffensive.
The Observatory said government forces brought "large military convoys to Hama" and its outskirts.
It said "regime forces and pro-government fighters led by Russian and Iranian officers were able to repel" an attack northwest of Hama.
The monitor also said that the fighting was close to an area mainly populated by Alawites, followers of the same offshoot of Shiite Islam as the president.
The rebels launched their offensive in northern Syria on November 27, the same day a ceasefire took effect in the war between Israel and Hezbollah in neighbouring Lebanon.
Both Hezbollah and Russia have been key backers of Assad's government, but have been more recently mired in their own respective conflicts.
The United Nations on Wednesday said 115,000 people had been "newly displaced across Idlib and northern Aleppo" by the fighting.
Human Rights Watch warned the fighting "raises concerns that civilians face a real risk of serious abuses at the hands of opposition armed groups and the Syrian government".
Until last week, the war in Syria had been mostly dormant for years, but analysts have said violence was bound to flare up as it was never truly resolved.
Spearheading the rebel alliance is HTS, which is rooted in Syria's Al-Qaeda branch.
"HTS has had a lot of time and space and resources to organise itself and to prepare for this," said analyst Sam Heller, of the US-based Century Foundation think tank.
How the fighting unfolds now "depends on whether the Syrian government can regain its footing", Heller said.
"Opposition forces currently pushing south will likely get stuck somewhere in Syria's centre, when they run into really motivated and intractable loyalist resistance," he said.
"At that point, it will be a question of whether Damascus has the means to mount the type of scorched-earth counteroffensive I assume it would like to execute."
