Syrian rebels on Wednesday encircled the key central city of Hama "from three sides", a war monitor said, despite a counteroffensive launched by regime forces to retain control of the city.
Hama is strategically located in central Syria and, for the army, it is crucial to safeguarding the capital and seat of power Damascus.
The fighting around Hama follows a lightning offensive by rebels headed by the hardline Hayat Tahrir al-Sham (HTS) who in a matter of days wrested swathes of territory, most significantly Syria's second city Aleppo, from regime leader Bashar al-Assad's grasp.
The rebels "have surrounded Hama city from three sides, and are now present at a distance of three to four kilometres (1.9 to 2.5 miles) from it," the Syrian Observatory for Human Rights said.
The Britain-based Observatory, which relies on a network of sources in Syria, said the government forces were "left with only one exit towards Homs to the south".
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 HTS, Abu Mohammad al-Jolani, on Wednesday visited Aleppo's landmark citadel.
Images posted on the rebels' Telegram channel showed Jolani waving to supporters from an open-top car as he visited the historic fortress.
In Hama, 36-year-old delivery driver Wassim said the sounds were "really terrifying" and the continuous bombing was clearly audible.
"I'll stay home because I have nowhere else to flee to," he said.
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.
A military source cited by SANA had earlier reported "fierce battles" against the rebels in northern Hama province since morning, adding that "joint Syrian-Russian warplanes" were part of the effort.
The Observatory said government forces brought "large military convoys to Hama" and its outskirts in the past 24 hours.
"Dozens of trucks" loaded with tanks, weapons, ammunition and soldiers headed towards the city, it said.
It said "regime forces and pro-government fighters led by Russian and Iranian officers were able to repel" an attack northwest of Hama.
It said the fighting was close to an area mainly populated by Alawites, followers of the same offshoot of Shia Islam as Assad.
The rebels launched their offensive on November 27, the same day a ceasefire took effect in the war between Israel and Hezbollah in neighbouring Lebanon.
The offensive is reportedly in response to Russian and Iranian-led strikes on Syria's northwest in recent weeks.
Both Hezbollah and Russia have been key backers of Assad's government, but have been more recently mired in their own respective conflicts.
Russia, Iran and Turkey are in "close contact" over the conflict in Syria, Moscow said on Wednesday.
While Russia and Iran both back Assad, Turkey has backed the opposition.
The United Nations on Wednesday said 115,000 people have been "newly displaced across Idlib and northern Aleppo" by the fighting.
Turkey meanwhile warned that it may be too soon to expect large-scale returns to Aleppo from among the three million Syrian refugees currently on its soil.
"To those who say they wish to go back now we say, 'wait, it's not safe for the moment'," said Turkish Interior Minister Ali Yerlikaya.
The Observatory says the violence has killed 704 people, mostly combatants but also 110 civilians.
Human Rights Watch on Wednesday warned that the fighting "raises concerns that civilians face a real risk of serious abuses at the hands of opposition armed groups and the Syrian government".
Rights groups including HRW have since the start of the war documented violations of human rights on both sides, including what could amount to "crimes against humanity" by Syrian regime forces.
Until last week the war in Syria had been mostly dormant for several years, but analysts have said violence was bound to flare up as it was never truly resolved.
"Many policymakers thought, well, Assad won, there is no war," said Rim Turkmani, director of the Syria Conflict Research Programme at the London School of Economics.
But "we've been worrying about this for years, that the fact that there is no intense violence doesn't mean that the conflict is over," she told AFP.
While the rebels may have advanced swiftly, it does not mean they will have the capacity to hold onto the territory they have captured.
"It's very well organised, very ideologically driven," Turkmani said.
"However, they spread very quickly and very thin. And I think very quickly they're going to realise it's beyond their capacity to maintain these areas and, most importantly, to govern them."