Reuters/Beirut

Islamic State fighters drove Syrian government forces from a town in the west of the country on Sunday, as fighting looked set to intensify despite a flurry of international diplomacy and talks between regional rivals.

The jihadists' 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 Islamic State.

The fighting tempered any expectation of progress towards a political solution to the four-year 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.

Washington said last week it would for the first time station ground troops in Syria to advise and assist rebels fighting Islamic State.

Talks between world powers in Vienna meanwhile adjourned with calls for a nationwide ceasefire but key differences remained between rivals backing opposing sides.

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

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

Islamic State confirmed the advance, which brought it within 20 km (13 miles) of the north-south highway linking Damascus to Syria's other main cities Homs, Hama and Aleppo.

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

Government and Russian air strikes have been targeting Islamic State fighters near an airbase they have long besieged east of Aleppo city.

Islamic State also faces an offensive launched by a new US-backed rebel alliance in the northeast province of Hasaka, closer to its strongholds of northern Syria and northwestern Iraq.

 Regional rivalry

 The United States is to step up its fight against Islamic State in both countries, having decided to station special forces in Syria to support rebels fighting against the jihadists, position more U.S. jets in Turkey and expand air strikes.

Air strikes by Turkish and US- aircraft in Syria on Saturday killed more than 50 Islamic State militants, Turkey's state-run Anadolu Agency said on Sunday.

Separately from the fight against Islamic State, Turkey and other Sunni regional powers including Saudi Arabia are backing insurgents fighting against President Bashar al-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.

Warring Syrian sides said on Sunday that despite international efforts to bring them to the negotiating table and end the conflict, fighting looked set to continue and intensify.

Before any political progress can be made, and ahead of new international talks set for within two weeks, rebels and government forces will both look to make gains on the ground and increase their leverage, several Western military officers attending a security conference this week in Bahrain said.

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.

A prominent rebel leader meanwhile said talks in Vienna on Friday had borne little fruit.

"If Russian-Iranian stubbornness continues, the situation will head toward escalation," warned Bashar al-Zoubi, head of the political office of the Yarmouk Army, a rebel group operating under the banner of the moderate Free Syrian Army.

Iran and Russia have increased military support for Assad, stepping up their presence in the country.

Saudi Arabia's Foreign Minister Adel al-Jubeir suggested repeatedly in the past week that in the face of Tehran and Moscow shoring up Assad, support to the Syrian opposition would intensify.

At the rare regional talks on security in Manama, he said Riyadh was considering intensifying support to moderate Syrian rebels by providing them with "more lethal weapons", but gave no further details.

As the conflict drags on, the United States has warned its former Cold War foe Russia could get bogged down in the Syrian "quagmire", alienating Sunni Muslims as Moscow is seen to be in league with Assad and Iran.