By Angus McDowall and Tom Perry
BEIRUT (Reuters) - Syrian rebels advanced to within a few kilometers of the government-held city of Hama on Wednesday in a major assault in the western region of Syria critically important to President Bashar al-Assad, a war monitor reported.
The offensive, spearheaded by Islamist militants, was launched on Tuesday after an attack in the capital Damascus, where heavy fighting persists, showing the lingering threat posed by rebels even as Assad enjoys the military upper hand.
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The Hama offensive also includes Free Syrian Army (FSA) rebels who had agreed to a truce in December brokered by Russia and Turkey, underlining the bleak prospects for U.N.-backed peace talks that are due to reconvene in Geneva on Thursday.
The Syrian Observatory for Human Rights reported the latest rebel advance on Hama. The Syrian army could not immediately be reached for comment.
A Syrian military source told Reuters the army was sending reinforcements against the rebel thrust, adding that the insurgents had mobilized large numbers for the assault.
A rebel commander told pro-opposition Orient TV that insurgents planned to open yet more fronts.
The Observatory, a Britain-based war monitor, said insurgents had captured the towns of Soran, 20 km (12 miles) north of Hama, Khattab, 10 km northwest of Hama, and al-Majdal, 6 km west of Khattab.
The assault on Soran began on Tuesday with two suicide bombers being deployed in addition to rockets and artillery.
The areas of Hama province targeted in the latest assault form part of Syria's pivotal western region where Assad has shored up his rule during the six-year-long war with crucial military support from Russia and Iran.
"There are fierce battles between the two sides," the military source said.
The attack is being led by Tahrir al-Sham, an alliance of Islamist factions dominated by a group that was formerly al Qaeda's official affiliate in the Syrian war.
"The battle - praise God - has been prepared for a long time and all capacities have been prepared for it so that we can wage a long battle," an FSA commander identified as a lieutenant in the Ezza Army group told Orient TV.
The rebels have been on the backfoot since Russia deployed its air force to Syria in 2015 to bolster Assad. They suffered their worst setback of the six-year war in December when government forces seized eastern Aleppo from the opposition.
Syrian government forces, supported by Iranian-backed militias, have been pressing their military edge despite the December truce, winning back more areas including one near Damascus that is the source of the capital's water supply.
The escalation in Hama province follows two rebel assaults on government-held areas in Jobar in Damascus, launched by opposition fighters from Eastern Ghouta, an opposition stronghold east of the capital.
State media said on Wednesday the army was waging fierce clashes with insurgents in northern Jobar and that the air force was pounding insurgents and their supply lines in the area.
The Ezza Army commander said the rebel attack in Hama was coordinated with the one in Damascus. He also said that more attacks would be launched "in a number of other areas".
The increase in fighting between the rebels and the army came in the run-up to new peace talks in Geneva.
But the obstacles facing peacemaking were on display last week when Turkey-backed rebel groups shunned talks in Kazakhstan aimed at firming up the December ceasefire and criticized Russia for failing to get its allies to adhere to the truce.
(Reporting by Tom Perry; editing by Mark Heinrich)