Syrian state media said militants launched a major attack on government forces in Idlib on Thursday that Russia's defense ministry said killed up to 40 Syrian soldiers, though the account was disputed by a rebel official and war monitor.
Idlib is the last rebel-held swathe of territory in the country and hundreds of thousands of people in the area have fled in recent weeks amid heavy air strikes by Russian and Syrian forces.
Russia's defence ministry said that militants had seized two settlements in one of the offensives, which began on Wednesday, forcing Syrian army troops to abandon some of their positions in the southeast of the so-called Idlib de-escalation zone under rocket fire.
Syrian state media made no mention of army casualties but said an attack that included car bombs and heavy gunfire early on Thursday had forced some of its forces to re-deploy.
Naji al-Mustafa, spokesman for the National Liberation Front coalition of rebel groups, disputed the Syrian media and Russian government accounts and said no such assault had taken place on Wednesday or Thursday.
Mustafa said attacks had been carried out against government forces earlier in the week as a response to stepped up strikes against them in Idlib, but the area had seen no operations over the past 24 hours.
The Syrian Observatory war monitor also said there was no attack on government forces in Idlib on Thursday but that it had recorded about 400 Russian and Syrian airstrikes on the area since Wednesday, part of a stepped up assault since December.
Russia, which is helping President Bashar al-Assad's forces, said Syrian government forces had killed up to 50 militants and wounded up to 90.
Moscow said the attackers were from different groups, including the Islamic Party of Turkestan and Tahrir al-Sham, the latest incarnation of the former Nusra Front that was part of al Qaeda until 2016.
The militants were equipped with pickup trucks, armoured personnel carriers, tanks and heavy machine guns, it said.