Syrian war planes on Friday bombed Islamic State fighters trying to advance into the northeastern city of Hasaka in an offensive that is adding to the pressure on president Bashar al-Assad following recent defeats elsewhere.
The Syrian Observatory for Human Rights, which monitors the war, also reported fierce battles between Islamic State militants and the army backed by allied militia on the city's southern outskirts.
Islamic State this week launched a big offensive targeting the government-held portion of Hasaka city, which is divided into zones run separately by the state and a Kurdish administration that has a well-organised militia.
The attack follows heavy defeats this last month for Islamic State in the northeast by the Kurdish YPG militia. With air support from a US-led alliance, the YPG has driven the jihadists from swathes of the surrounding area.
Islamic State has used around a dozen suicide car bombers in the attack on Hasaka city this week, and has seized two government-held positions south of the city - a prison under construction and a power installation.
An Islamic State news bulletin said the group’s fighters were 1 km from the city, with only one army position separating them from its southern entrance. A Syrian military source said, however, that Islamic State’s attack had been repelled, describing the situation as excellent.
The Observatory said dozens of combatants had been killed on both sides in this week’s fighting.
Dr Assad, having lost territory to insurgents in the northwest, east and south in recent months, is under greater military pressure than at any point in the four-year-long conflict.
Islamic State’s capture of Palmyra from government forces last month marked the first time the group had seized a large population directly from government control.
The war is a focal point for a regional struggle between conservative Gulf Arab Sunni states and Shi'ite Islamist Iran.
The setbacks have drawn repeated Iranian statements of support for Dr Assad. But as yet there has been no reversal of the insurgent gains.
Islamic State has turned its sights on government-held Hasaka because it views the Syrian army as a softer target than the YPG in the northeast, according to assessments given by the YPG and the Observatory.
The YPG spokesman told Reuters earlier this week that government forces would not be able to hold out if Islamic State was serious about seizing Hasaka. That raises the prospect of a new front between the YPG and Islamic State.