Islamic State is fighting an intense battle to seize a new base in eastern Syria as it loses ground in its most important stronghold, Mosul.
Isis fighters launched a big attack over the weekend on the Assad regime-held city of Deir Ezzor, disabling its airport.
The group managed to kill a number of regime fighters before the Syrian army and supporting militias staged a counter-attack.
The advance by the group is the second against the regime in Syria since it started to lose ground in and around Mosul, with leaders thought to be looking to find new secure positions in the country as it is gradually pushed out of Iraq.
The first attack began on Friday night at the airport, according to the local activist-run news site Deir Ezzor 24, and continued into Saturday morning, with 30 people on both sides, including civilians, reported killed.
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The regime managed to regroup but Isis attacked again on Sunday, seizing areas around the hospital.
About 200,000 civilians are said to be trapped in regime-held areas of the city, which are surrounded by Isis. They are getting supplies through the airport and by air drops, though conditions are said to be worsening.
Last month Isis retook Palmyra, further west, in another sign that the group is facing weaker resistance in the country as the regime and its Iranian and Russian backers focus on fighting other rebel groups in areas that it deems more strategically important.
Offensives against Isis on other fronts in Syria have also slowed: against the Isis “capital” Raqqa by the Kurdish-led Syrian Democratic Forces, backed by the US-led coalition and its air forces, and against the northern town of al-Bab by Turkish-backed Free Syrian Army groups.
The Assad regime has always made clear that it regards the fight against the mainstream rebel groups and the former al-Qaeda affiliate known as Fateh al-Sham Front as its first priority.
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Rebel groups announced today that they would attend talks with the regime in Astana, Kazakhstan, scheduled for next week, that have been prepared by Russia and Turkey.
The United States also confirmed over the weekend that it had been invited to attend.
Russia hopes to use the talks to cement its position as the key regional power-broker. All sides say a ceasefire, perhaps leading to a broader deal for Syria, would enable all the countries involved to focus on getting rid of jihadist groups such as Fateh al-Sham Front and Isis.
The rebel groups said they would be concentrating on finding terms for a broader ceasefire than that agreed at present, which allows the regime to continue attacking areas where it believes Fateh al-Sham fighters to be present.