Islamic State (IS) fighters have launched simultaneous attacks against the Syrian government and Kurdish militi, moving back onto the offensive after losing ground in recent days to Kurdish-led forces near the capital of their "caliphate".

After recent losses to the Kurdish forces backed by US-led air strikes, IS sought to retake the initiative with attacks on the Kurdish-held town of Kobani at the Turkish border and government-held areas of Hasaka city in the northeast.

In a separate offensive in the multi-sided Syrian civil war, an alliance of rebels in the south of the country also launched an attack with the aim of driving government forces from the city of Deraa.

The attacks by IS follow two weeks that saw the Kurds advance deep into the hardline group's territory, to within 30 miles of its de facto capital Raqqa, hailed as a success by Washington.

The US and European and Arab allies have been bombing IS since last year to try and defeat the group which a year ago proclaimed a caliphate to rule over all Muslims from territory in Syria and Iraq.

The US-led campaign faced serious setbacks last month when IS seized cities in both Syria and Iraq. The latest Kurdish advance in Syria has again shifted momentum against the jihadists, but IS fighters have adopted a tactic of advancing elsewhere when they lose ground.

The group said its fighters had seized al-Nashwa district and neighbouring areas in the southwest of Hasaka, a city divided into zones of government and Kurdish control. Government forces had withdrawn towards the city centre, it said in a statement.

Syrian state TV said IS fighters were expelling residents from their homes in al-Nashwa, executing people and detaining them. Many IS fighters had been killed, it said, included one identified as a Tunisian leader.

The Syrian Observatory for Human Rights, which tracks the war, said IS had seized two districts from government control.

Government-held parts of Hasaka are one of President Bashar al-Assad's last footholds in the northeast region bordering Iraq and Turkey, territory mainly governed by Kurds since Syria descended into civil war in 2011.

The separate IS attack on Kobani began with at least one car bomb in an area near the border crossing with Turkey. Kurdish officials and the Observatory said IS fighters were battling Kurdish forces in the town itself.

Kobani was the site of one of the biggest battles against IS last year. Kurdish forces known as the YPG, backed by US air strikes, expelled the fighters in January after four months of fighting.

A doctor in Kobani, Welat Omer, said 15 people had been killed and 70 wounded, many of them seriously. Some had lost limbs. Some of the wounded had been taken to Turkey.

Around 50 people fled to Kobani's Mursitpinar border gate with Turkey after the attack, seeking to cross the border.

IS also killed at least 20 Kurdish civilians in an attack on a village south of Kobani, the Observatory reported.

The USs has rejected the idea of working with Assad in the war against IS.

Elsewhere in Syria, Assad's government has faced increased military pressure since March, losing ground in the northwest, the south and the centre of the country, where IS seized the city of Palmyra last month.

Assad's control is now mainly confined to the major population centres of western Syria, where he has sought to shore up his grip with the help of Lebanon's Hezbollah Shi'ite militia, his main allies.