Beirut: A rebel mortar attack killed four civilians on Sunday in the Syrian capital as government forces battled Islamic State group jihadists in the northern city of Hasakeh, a monitor said.

The Syrian Observatory for Human Rights said a mortar round struck Revolution Street in central Damascus, killing four people and wounding 13.

Syria's state-run SANA news agency also reported the attack but said three people died.

In Hasakeh, regime troops pressed an offensive they launched on Saturday to dislodge IS from districts they have seized in the southern part of the city.

The Britain-based Observatory, which has been monitoring the Syrian conflict since it erupted four years ago, said the troops and militants were locked in fierce fighting Sunday.

The jihadist group blew up an explosive-rigged vehicle in Hasakeh, the monitor said, without mentioning casualties.

IS seized two neighbourhoods in southern Hasakeh on Thursday in a new attempt to seize the provincial capital, causing tens of thousands of people to flee, according to the United Nations.

Meanwhile, government reinforcements have poured into Hasakeh, and Kurdish fighters, who control the city jointly with regime loyalists, have also taken part in the battles.

IS also tried to seize Hasakeh last month, but was pushed back by government forces.

The Observatory, which relies on a network of sources inside Syria, also reported on Sunday that the number of civilians killed by IS in the border town of Kobane had risen to 223.

The IS killing spree began after the extremists entered the Kurdish town at dawn on Thursday.

Kurdish fighters repelled the jihadists Saturday, and bodies, including of women and children -- some shot at close range -- were found in homes and in the streets.