BEIRUT: ISIS has expanded its presence in central Syria, seizing control of a town that lies near a highway leading to the capital, Damascus, activists and the group said Sunday.

The militants’ advance came even as Russian warplanes and Syrian forces supported by them stepped up assaults against insurgents in west and northwest Syria, and the United States separately sought to increase pressure on ISIS.

The fighting tempered any expectation of progress toward a political solution to the nearly 5-year-old civil war, with warring sides and their foreign backers refusing to back down in a conflict where the world’s major military powers except China are directly involved.

In a fierce assault that began by detonating two suicide car bombs, ISIS militants took the town of Maheen in the southwest of Homs province from government forces, a group monitoring the war said.

ISIS Internet radio broadcast Al-Bayan said the group took control of large arms depots in the town.

Some 50 fighters on the government side were killed, and clashes raged afterward on the outskirts of a nearby mostly Christian town, Sadad, the Syrian Observatory for Human Rights said.

ISIS confirmed the advance, which brought it within 20 kilometers of the north-south highway linking Damascus to Syria’s other main cities Homs, Hama and Aleppo.

The Observatory’s Rami Abdel-Rahman said the attack might have been a response to pressures the group is under elsewhere including in northern province Aleppo.

Government and Russian airstrikes have been targeting ISIS fighters near an air base they have long besieged east of Aleppo city.

On the other side of Syria in the northeastern province of Hassakeh, ISIS is facing a new offensive launched by a recently formed U.S.-backed rebel alliance.

The Observatory reported fierce fighting between the rebel alliance, including the Kurdish YPG militia, and ISIS fighters in the area of Al-Houl near the Iraqi border.

The U.S. says it will step up its fight against ISIS in Syria and Iraq, having decided to station special forces in Syria to support rebels fighting the militants, and to position more U.S. jets in Turkey and expand airstrikes.

Airstrikes by Turkish and U.S. aircraft in Syria Saturday killed more than 50 ISIS militants, Turkey’s state-run Anadolu Agency said Sunday.

Separately from the fight against ISIS, Turkey and other Sunni regional powers including Saudi Arabia are backing insurgents fighting against President Bashar Assad.

Russia’s air campaign in Syria in support of ally Assad has mainly targeted those insurgents, as Moscow and Tehran seek to prop up the Syrian government.

A Syrian army source said that Saudi Arabia and Turkey’s continued support for rebels meant violence would not abate.

“The battle is still a long one. All the time support does not stop to the terrorists from regional states, at the forefront of them Saudi Arabia and Turkey,” he said.

Syria’s government refers to all rebels fighting it as terrorists.