Islamist rebels and their allies were advancing towards the key Syrian city of Hama on Tuesday, according to the UK-based Syrian Observattory for Human Rights (SOHR).
Militants with the Hayat Tahrir al-Sham (HTS) group were fighting in what SOHR called the "most violent" clashes with government troops since launching their surprise offensive last week.
Syria and Russia strike Hama region
"Clashes have erupted in the northern Hama countryside, where rebel factions managed to seize several cities and towns in the last few hours," said the monitor, which relies on a large network of sources inside Syria.
The Associated Press reported that the rebels were just 10 kilometers (6 miles) away from the city.
Syrian government forces were reportedly preparing for a counter-attack, according to the Syrian state news agency SANA.
The SOHR added that "Syrian and Russian air forces carried out dozens of strikes on the area."
Russian President Vladimir Putin has been a longtime ally of Syrian President Bashar al-Assad, and has regularly provided material support since Assad's clampdown on protests sparked the Syrian Civil War in 2011.
Russia began intervening directly in 2015 with airstrikes when the extremist "Islamic State" (IS) entered the conflict.
Hama is key link between Aleppo and Damascus
Syrian state news agency SANA also reported air strikes on Hama province and rebel bastion Idlib in the northwest. Hama is considered a strategically important city because it connects Aleppo, which HTS swept into last week, and the capital Damascus.
The area is home to the Alawite community from which Assad hails, and thus a rebel takeover of Hama would "pose a threat to the regime's popular base," SOHR director Rami Abdel Rahman said.
According to the United Nations, some 50,000 people have been displaced and hundreds, mostly fighters, have been killed since the conflict touched off again in November.
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