The Syrian air force hit an Isis-controlled air base in Raqqa province on Tuesday, killing more than 140 militants, state media said, striking the jihadist group in its Syrian stronghold a week after it seized Palmyra from the government.
The city of Raqqa is seen as the de facto capital of the "caliphate" Islamic State has declared in Syria and Iraq. The group has made notable gains in both countries this month, capturing both Ramadi in Iraq and Palmyra in Syria.
It was not possible to independently verify the reported attack on the Tabqa air base, which Isis seized in August, killing scores of captive government soldiers.
The Syrian Observatory for Human Rights, a group monitoring the war, said a US-led coalition waging a separate campaign against Isis had targeted the same area on Monday.
Isis's takeover of Palmyra, also known as Tadmur, marked the first time the group had seized a city directly from government control. The other population centres it holds were mostly taken from rival insurgent groups in Syria's civil war.
Palmyra is strategically significant for reasons, including its location at a crossroads of highways in the centre of Syria leading both to Homs and Damascus, President Bashar al-Assad's seat of power.
It also has a UNESCO World Heritage site. Isis's capture of the city has raised fears its 2,000-year-old, Roman ruins could meet the same fate as ancient sites destroyed by the ultra-radical group in Iraq.
The Syrian antiquities chief told Reuters that Palmyra's ruins were so far undamaged. "The historic city is fine. There is no damage so far," said Maamoun Abdulkarim, citing contacts with people on the ground.
However, he said he was still afraid the jihadist militants would blow up ruins, including tombs and the Temple of Bel, which could be viewed as idolatrous in its puritanical vision of Islam.
A short video posted by an account supportive of Isis on YouTube on Tuesday claimed to show Palmyra after the jihadists took control.
The footage, mostly filmed without sound and people, showed the Palmyra's ancient citadel, columns, colosseum, buildings and walls. One image showed black smoke rising behind ancient ruins but it did not appear that any of the historical sites had been damaged by the week of fighting.
The United States has ruled out the idea of partnering with Assad in the fight against Isis. It regards Assad as part of the problem and says he should leave power.
A Syrian army source said Isis was trying to move combatants from its bastions in Raqqa and Deir al-Zor in eastern Syria to Palmyra.
He said Isis would "certainly try" to make further gains after Palmyra, but added: "We are not at all worried. It will not be able to advance towards the west."
"In Palmyra, there are attempts to call up terrorists from Raqqa, Deir al-Zor," said the source.
"There is notable (Isis) activity, more than before, because after what happened in Tadmur, it will not halt its attempts to expand," he said.
Isis sustained heavy losses in an army attack near Palmyra on Monday, the source added.
The fall of Palmyra followed numerous attacks in recent months by Isis on government-held areas in northern, southern and central Syria. – Reuters, May 26, 2015.