* IS pounded again in northern Syria

* Group presses attack on strategic border town

* Turkey says its troops could be used in Syria (Adds monitoring group on IS casualties, 9th paragraph)

By Mariam Karouny and Jonny Hogg

BEIRUT/MURSITPINAR, Turkey, Sept 27 (Reuters) - New U.S.-led air strikes against Islamic State fighters failed to stop them from pressing their assault on a strategic Syrian town near the Turkish border on Saturday, hitting it with shell fire for the first time.

The U.S. Central Command said the air strikes destroyed an IS building and two armed vehicles near the border town of Kobani, which the insurgents have been besieging for the past 10 days.

It said an airfield, garrison and training camp near the IS stronghold of Raqqa were also among the targets damaged in seven air strikes conducted by the United States, Saudi Arabia, Jordan and the United Arab Emirates, using fighter planes and remotely piloted aircraft.

Three air strikes in Iraq destroyed four IS armed vehicles and a "fighting position" southwest of Arbil, Centcom said.

The United States has been carrying out strikes in Iraq since Aug. 8 and in Syria, with the help of Arab allies, since Tuesday, in a campaign it says is aimed at "degrading and destroying" the Islamist militants who have captured swathes of both countries.

A day after the UK parliament voted to allow British warplanes to attack IS in Iraq, two British fighter jets flew a mission over the country, the Ministry of Defense said, adding they had gathered intelligence but did not carry out air strikes.

IS, which swept across northern Iraq in June, has proclaimed an Islamic "caliphate," beheaded Western hostages and ordered Shi'ites and non-Muslims to convert or die. Its rise has prompted President Barack Obama to order U.S. forces back into Iraq, which they left in 2011, and to go into action over Syria for the first time.

The Syrian Observatory for Human Rights, a monitoring group that supports opposition forces fighting Syrian President Bashar al-Assad, said Saturday's air strikes set off more than 30 explosions in Raqqa.

Rami Abdulrahman, who runs the British-based Observatory, said 23 Islamic State fighters were killed. He said the heaviest casualties were inflicted in attacks on an airport.

But the monitoring group said IS was still able to shell eastern parts of Kobani, wounding several people, in a sign that its fighters were drawing closer. The insurgents' offensive against the Kurdish town, also known as Ayn al-Arab, has prompted around 150,000 refugees to pour across the border into Turkey since last week.

ERDOGAN SHIFT

Turkish President Tayyip Erdogan signaled a shift in Ankara's position by saying for the first time that Turkish troops could be used to help set up a secure zone in Syria, if there was international agreement to establish one as a haven for those fleeing the fighting.

Turkey has so far declined to take a frontline role in the U.S.-led coalition against IS, but Erdogan told the Hurriyet newspaper: "The logic that assumes Turkey would not take a position militarily is wrong."

He said negotiations were under way to determine how and by which countries the air strikes and a potential ground operation would be undertaken, and that Turkey was ready to take part.

"You can't finish off such a terrorist organization only with air strikes. Ground forces are complementary ... You have to look at it as a whole. Obviously I'm not a soldier but the air (operations) are logistical. If there's no ground force, it would not be permanent," he said.

Turkish officials near the Syrian border said IS fighters battling Kurdish forces for Kobani sent four mortar shells into Turkish territory, wounding two people.

One of the shells hit a minibus near Tavsanli, a Turkish village within sight of Kobani. A large hole was visible in the rear of the vehicle.

"Two people were injured in the face when the minibus was hit. If they'd been 3 meters (10 feet) closer to the car, many people would have died," said Abuzer Kelepce, a provincial official from the pro-Kurdish party HDP.

Heavy weapons fire was audible, and authorities blocked off the road towards the border.

"The situation has intensified since the morning. We are not letting anyone through right now because it is not secure at all. There is constant fighting, you can hear it," the official said.

Kobani sits on a road linking north and northwestern Syria. IS militants were repulsed by local forces, backed by Kurdish fighters from Turkey, when they tried to take it in July, and that failure has so far prevented them from consolidating their gains in the region.

COALITION WIDENS

Syria's government, which in the past accused its opponents of being Western agents trying to topple Assad, has not objected to the U.S.-led air strikes, saying it was informed by Washington before they began.

It too has carried out air strikes across the country, including in the east, and its ground forces have recaptured the town of Adra, northeast of Damascus, tightening Assad's grip on territory around the capital.

But Russia has questioned the legality of U.S. and Arab state air strikes in Syria because they were carried out without the approval of Damascus, Moscow's ally.

General Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, said on Friday that this week's strikes in Syria had disrupted Islamic State's command, control and logistics capabilities. But he said a Western-backed opposition force of 12,000 to 15,000 would be needed to retake areas of eastern Syria controlled by the militants. (Reporting by Mariam Karouny; additional reporting by Michele Kambas; writing by Mark Trevelyan; editing by Jason Neely)