Islamic State militants battled rival insurgent groups on Wednesday north of the city of Aleppo, where officials say the Syrian army is preparing an offensive of its own backed by Iranian soldiers and Russian jets.
A rebel fighter and a group monitoring the war said Islamic State fighters took control of part of the towns of Ahras and Tel Jabin, about 12 km (8 miles) north of Aleppo, before being pushed back.
Gains by Islamic State north of Aleppo will threaten the supply lines of rival rebels inside the city, which is divided between insurgents and government forces. The Syrian Observatory for Human Rights said the road used by Aleppo residents heading north to the Turkish border remained closed on Wednesday.
"Today there are fierce battles between us and Daesh in Ahras, Tel Jabin, and rural northern Aleppo," said Hassan Haj Ali, head of the Liwa Suqour al-Jabal rebel group, using another name for Islamic State.
His unit is one of several foreign-backed insurgent forces which find themselves fighting Islamic State on the ground, at the same time as they are bombed by Russian jets and are bracing for a broad ground offensive by the army.
"There are mobilisations by the regime in most parts of Aleppo, particularly in Bashkoy," he said, referring to another town north of Aleppo, which before Syria's civil war began in 2011 was the country's biggest city and a major commercial and industrial centre.
"There were advances (by Islamic State) at dawn but we were able to recover Ahras entirely. There are battles in Tel Jabin," said Ali, speaking to Reuters via an internet messaging system.
Iran has sent thousands of additional troops into Syria in recent days to bolster one offensive that is underway in Hama province and in preparation for another in the Aleppo area, two senior regional officials told Reuters.
In another sign of Iran's central role in support of President Bashar al-Assad, a team of Iranian lawmakers arrived in Damascus on Wednesday, and pictures circulated on social media showed the head of the Revolutionary Guards' Quds Force, Qassem Soleimani, in western Syria.
Soleimani was pictured in a wooded area of northern Latakia province addressing Iranian officers and Hezbollah fighters with a microphone, wearing dark clothes as he spoke to the men in camouflage.
"This leak at this time is deliberate and part of managing the battle in which the Russian Sukhois (warplanes) are taking part," said Salem Zahran, a Lebanese media pundit with close ties to Hezbollah and the Syrian government.
Two senior Revolutionary Guards officers were killed fighting Islamic State in Syria on Monday, Iran's Tasnim news agency reported, although it did not say where. Another senior Guards commander was killed last week, as was a top Hezbollah commander.
Supported by two weeks of air strikes, the Syrian army and its allies have been fighting insurgents in northern Hama province, and neighbouring Idlib and Latakia provinces, trying to reverse rebel gains over the summer which had threatened the coastal heartlands of Assad's Alawite minority.
Assad's foreign opponents appear to be stepping up support for rebel groups in response to the Russian-Iranian intervention. Rebels say they have plentiful supplies of U.S.-made anti-tank missiles that are helping them to hold off ground attacks.
Two rebel commanders said on Tuesday they have stationed a dozen TOW anti-tank missile platforms supplied from abroad along a 30 km (20 miles) defensive line in Hama province in an effort to contain the army advance.
Activists said supplies of weapons had been stepped up since the Russian air strikes began on Sept. 30. The missiles have been widely seen as important to rebel advances earlier this year that had put Assad under pressure.
"We have an excellent supply of missiles," Fares Bayoush, head of the Fursan al-Haq rebel group, told Reuters from Syria. "We will, God willing, move to attack, not just defence."
Russia has stepped up its air strikes in recent days, announcing on Tuesday it had carried out 88 missions in the previous 24 hours, one of the heaviest days of bombardment of its campaign so far.
Moscow's intervention means Russian and U.S. jets are flying combat missions over the same country for the first time since World War Two, raising fears of possible accidental confrontation.
Russia says it has asked Washington to discuss coordination of military efforts, but Foreign Minister Sergei Lavrov said on Tuesday the United States had declined to send a high-level military delegation to Moscow for discussions.
Syrian state media said on Wednesday the army had also launched a fresh military operation against rebel-held areas east of Damascus, including Jobar and Harasta, controlled by non-Islamic State rebel groups including Jaish al-Islam.