Islamic State fighters tightened their siege of a town on Syria's border with Turkey on Friday despite US-led air strikes aimed at defeating the militants in both Syria and Iraq, in a coalition which has now drawn widespread European support.

Britain, Washington's closest ally in the wars of the last decade, joined an alliance after weeks of weighing its options. Britain's parliament voted 542 to 43 to back Prime Minister David Cameron's decision to take part in air strikes in Iraq.

Belgium's parliament also voted 114 to 2 to take part and Denmark said it would send planes. Six Belgian F-16s took off for a staging post in Greece even before the vote.

"This is not a threat on the far side of the world. Left unchecked, we will face a terrorist caliphate on the shores of the Mediterranean and bordering a NATO member, with a declared and proven intention to attack our country and our people," Cameron told British lawmakers.

Until this week France was the only Western country to answer President Barack Obama's call to join the campaign. Since Monday, Australia and the Netherlands have also joined. On Friday Germany expressed support for the mission despite saying it would not send aircraft of its own.

Obama has sought international support for a military coalition against Islamic State, a powerful force in Syria which swept across northern Iraq in June, slaughtering prisoners and ordering Shi'ites and non-Muslims to convert or die.

The campaign has brought Washington back to the battlefield in Iraq that it left in 2011 and into Syria for the first time after avoiding involvement in a war that began the same year.

The coalition also includes several Arab states, all led by Sunni Muslims alarmed at the rise of Islamic State.

Islamic State has emerged as the most powerful Sunni militant group battling Shi'ite-backed governments in Iraq and Syria. The militants are also fighting rival Sunni rebel groups in Syria and Kurds in both Syria and Iraq, countries facing complex, multi-sided civil wars in which nearly every state in the Middle East has a stake.

French public support for the mission surged this week after the beheading of a French tourist in Algeria by captors who said it was retaliation for French participation in strikes in Iraq.

Paris said it might also join US strikes in Syria although there was no plan yet to do so. European countries have so far agreed only to strike targets in Iraq, where the government has asked for help, and not in Syria, where President Bashar al-Assad has not given permission, although he has not objected.

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

"It's very important that such cooperation with Syrian authorities is established, even now that it's an accomplished fact," Russian Foreign Minister Sergei Lavrov told reporters on the sidelines of the U.N. General Assembly in New York.

'WE'RE AFRAID'

More than a month since the US military began striking Islamic State targets in Iraq, and four days since it extended the campaign into Syria, there are signs fighters are lowering their profile in areas they control to become a harder target.

General Martin Dempsey, chairman of the US Joint Chiefs of Staff, said this week's strikes in Syria had disrupted Islamic State's command, control and logistics capabilities.

But Dempsey 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. US assessment teams have arrived in Saudi Arabia to map out a program expected to train more than 5,000 opposition fighters in the first year.

"We have to do it right, not fast," Dempsey said.

The air campaign has yet to halt Islamic State's advance in Syria, where fighters have laid siege to a Kurdish town on the Turkish border, sending 140,000 refugees across the frontier since last week in the fastest exodus of the three-and-a-half-year-old civil war.

The main battle in northern Syria has been visible from across the border in Turkey. The boom of artillery and bursts of machine-gun fire echoed across the area and at least two shells hit a vineyard on the Turkish side of the border, though there were no immediate reports of casualties inside Turkey.

"We're afraid. We're taking the car and leaving today," said vineyard owner Huseyin Turkmen, 60, as small arms fire rang out in the Syrian hills just to the south.

Islamic State fighters appeared to have taken control of a hill 10 km (6 miles) west of Kobani from where the YPG, the main Kurdish armed group in northern Syria, had been attacking them.

The British-based Syrian Observatory for Human Rights, which monitors the war, said Islamic State fighters had also taken control of a village around 7 km (4 miles) east of Kobani.

Kurdish forces said on Thursday they had pushed back the advance on Kobani, also known as Ayn al-Arab, but appealed for US-led air strikes on the insurgents' tanks and heavy weapons.

"The clashes are moving between east, west and south of Kobani ... The three sides are active," Idris Nassan, deputy foreign minister in the area's Kurdish administration, said by phone from the centre of the town.

"They are trying hard to reach Kobani. There is resistance here by YPG, by Kobani and some volunteers from north Kurdistan Turkish Kurds," he said. "Every girl, every young man, every man who is able to fight, to carry a gun, they are armed and they are ready to defend and fight."

NATO member Turkey has been conspicuously absent from the coalition against Islamic State, angering its Kurdish residents.

SYRIAN AIR STRIKES

The US military said its planes blew up four Islamic State tanks in eastern Syria and hit a number of targets in Iraq.

The Syrian Observatory monitoring group said one US-led strike in eastern Syria had killed an "important" Islamic State figure on a motorbike. It did not identify the victim.

Assad's Syrian government has not objected to the US-led campaign against some of his most powerful foes. Washington says it wants to defeat Islamic State without helping Assad remain in power and hopes other anti-Assad groups can fill the vacuum.

But while US planes have been striking Islamic State in eastern Syria, Assad's air force has been bombing other rebel groups in the west of the country, and his troops and allied Lebanese Shi'ite militia have advanced.

Syrian warplanes dropped projectiles including "barrel bombs" - oil drums filled with explosives - in Hama, Idlib, Homs and Aleppo provinces and around Damascus, the Observatory said.

Five people were killed when barrel bombs were dropped on al-Rastan city in Homs province and nine died in a barrel bomb attack east of the city of Aleppo, it said.

In Iraq, where the US strikes have gone on for far longer and Washington is supporting government efforts to advance, Islamic State militants are changing tactics, ditching conspicuous convoys in favour of motorcycles and planting their black flags on civilian homes to confuse target spotters.

Witnesses and tribal sources in Islamic State-controlled areas report fewer militant checkpoints to weed out "apostates" and less cell phone use.

Islamic State elements "abandoned one of their biggest headquarters in the village" when they heard the air strike campaign was likely to target their area, said a tribal sheikh from a village south of Kirkuk.

"They took all their furniture, vehicles and weapons. Then they planted roadside bombs and destroyed the headquarters," said the sheikh who declined to be identified.

Tribal and local intelligence sources said an air strike on Thursday near Bashir town, 20 km (12 miles) south of Kirkuk, had killed two local senior Islamic State leaders while they were receiving a group of militants from Syria and Mosul. Ongoing fighting makes it impossible to verify the reports.

Sheikh Anwar al-Assy al-Obeidi, the head of a large tribe in Kirkuk and across Iraq, told Reuters there were now fewer killings because fighters could not operate as openly.

"They were executing people like drinking water ... Now the air strikes are very active and have decreased the (militants') ability," Obeidi said. – Reuters