AMMAN (Reuters) -Syrian rebels said they had started pushing into the city of Hama on Thursday, but the army denied that as pro-government forces backed by Russian airstrikes fought to stave off a new insurgent victory and halt their lightning advance.

Rebel commander Hassan Abdul Ghany posted on social media that the insurgents had begun to penetrate Hama and were advancing along several routes towards its centre.

The Syrian army said there was no truth to reports rebels had entered any city district and that its forces remained stationed around Hama's outskirts behind impregnable defensive lines.

The rebels took the main northern city of Aleppo last week and have since pushed south from their enclave in northwest Syria, reaching a strategic hill just north of Hama on Tuesday and advancing towards the city's east and west flanks on Wednesday.

Hama has remained in government hands throughout the civil war, which erupted in 2011 as a rebellion against President Bashar al-Assad. Its fall to a revived insurgency would send shockwaves through Damascus and its Russian and Iranian allies.

The city lies more than a third of the way from Aleppo to Damascus and its capture would open the road for a rebel advance on Homs, the main central city that functions as a crossroads connecting Syria's most populous regions.

Inside Hama, the scene of an Islamist uprising that the Assad dynasty crushed in 1982, the internet was cut off and streets emptied on Wednesday according to a resident whose family remain in the city.

Hama is also critical to the control of two major towns with big minority religious communities, Muhrada, home to many Christians, and Salamiya where there are many Ismaili Muslims.

The return of full-blown civil war in Syria after years of frozen frontlines risks further destabilising a region ablaze from conflicts in Gaza and Lebanon.

At the height of the conflict a decade ago, regional and global powers backed rival forces across the country and the chaos created space for Islamic State to seize territory that it used as a launchpad for attacks around the world.

ADVANCE

The most powerful rebel faction is the militant Sunni Islamist Hayat Tahrir al-Sham (HTS), the former al Qaeda affiliate in Syria. Its leader Abu Mohammed al-Golani has pledged to protect Syria's religious minorities and has called on them to abandon Assad, but many remain fearful of the insurgents.

On Wednesday, Golani visited Aleppo's historic citadel, a symbolic moment for rebels who were driven out of the city in 2016 after months of siege and intense fighting, their biggest defeat of the war. Aleppo was Syria's biggest city before the war.

HTS and the other rebel groups are trying to consolidate their rule in Aleppo, bringing it under the administration of the so-called Salvation Government they established in their northwestern enclave.

Aleppo residents have said there are shortages of bread and fuel, and that telecoms services have been cut.

The rebel forces advancing on Hama have included a Turkey-backed insurgent coalition called the Syrian National Army, which holds a strip of territory along the Syrian-Turkish frontier, rebel sources said.

Turkey, which designates HTS as a terrorist organisation, has long been the biggest external backer of other rebel factions and its role will be critical to the future of any enlarged insurgent region in Syria.

Ankara has denied having taken part in the rebels' sudden sweep into Aleppo last week.

On the government side, Russia and Iran were crucial to Assad's success in recovering most Syrian territory and all main cities from 2015-2020, and they have sworn to help him again.

But Moscow has been focused on the war in Ukraine, while Iran's most important regional ally Hezbollah, which for years played a critical role shoring up Assad in Syria, has suffered heavy losses to Israel in Lebanon over the past two months.

Russian airstrikes across rebel-held areas of northern Syria have sharply intensified over the past week. Iran-backed militia groups from Iraq reinforced frontlines after bringing fighters across the border on Monday, Iraqi and Syrian sources said.