Three European nations — including Britain — joined the widening U.S.-led air campaign against Islamic State militants in Iraq on Friday, even as the group’s fighters renewed their attempt to overrun a strategic border city in Syria.

Britain’s entry seven weeks after the United States began carrying out strikes followed an overwhelming parliamentary vote to authorize attacks. Denmark and Belgium also opted to join the fight.

But as the coalition expanded, its constraints became clear. All three countries that authorized military action Friday are limiting their roles to Iraq. Meanwhile, Islamic State militants demonstrated that airstrikes have failed to slow their assault on critical positions in Syria.

Along the Turkey-Syria border, Islamic State fighters backed by artillery fire pushed toward the city of Kobane — known in Arabic as Ayn al-Arab — as Syrian Kurdish forces dug in for a key test of their strength.

The United States and its Arab allies broadened their campaign to targets in Syria this week after a drumbeat of U.S. strikes in Iraq since early August.

By an overwhelming majority, Britain's Parliament on Friday voted in favor of airstrikes against Islamic State militants in Iraq. (Reuters)

But no European ally has been willing to join the Syria campaign — raising the prospect that the Islamic State could try to use the country as a refuge.

“Simply allowing [the Islamic State] to retreat across an invisible border is no answer,” said Peter Hain, a member of Parliament and former cabinet minister, during Britain’s day-long debate.

British Prime Minister David Cameron, scarred by a humiliating defeat last year when he sought permission to launch strikes against the government of Syrian President Bashar al-Assad, did not try to win approval for attacks in Syria this time around.

Instead, he limited his proposal to Iraq, where he had a clear consensus, thanks to the Iraqi government’s request for Western help. No such invitation from Syria exists, and British opposition leader Ed Miliband has suggested he will not support widening the campaign without a U.N. resolution, which is unlikely to come.

Friday’s House of Commons vote endorsing Cameron’s plan to deploy six Tornado fighter jets to Iraq was lopsided, at 524 to 43.

Cameron’s justification

Still, there was opposition from the backbenches, both from hawks who wanted to go further and from doves who insisted that Britain had not learned the right lessons from more than a decade of combat in Iraq and Afghanistan.

But Cameron argued that ignoring the Islamic State was impossible, given the threat he said it poses to Britain.

The United States has led a series of airstrikes in Syria. A look at the campaign thus far.

“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 said as he opened the debate.

Cameron and others who support airstrikes were quick to differentiate Friday’s vote from the last time the British Parliament authorized military action in Iraq, in 2003. Cameron stressed that there would be no boots on the ground and said the air campaign would be marked more by “patience and persistence” than “shock and awe.”

The British contribution is modest, representing only a third the number of its jets that flew over Libya during the 2011 campaign against Moammar Gaddafi’s government. But it is similar to the commitment of other nations that have joined the coalition against the Islamic State, including France, the Netherlands and Australia.

While the British public was divided over joining the air campaign when the United States launched strikes, opinion has solidified in favor of the idea in recent weeks — especially since Islamic State militants executed two American journalists and a British aid worker. At least two other Britons are known to be held by the group and have appeared in Islamic State propaganda videos.

European counterterrorism officials have expressed deep concern that the Islamic State will try to carry out attacks on Western soil, perhaps employing some of the estimated 3,000 Europeans who have traveled to Iraq and Syria to fight with the organization.

Cameron suggested Friday that there would be a strong case for expanding Britain’s air campaign to Syria — but said that would require a separate parliamentary debate.

The battle for Kobane

Meanwhile, the battle in Syria rages. Gen. Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, said Friday that airstrikes by the United States and its Arab allies had disrupted the Islamic State’s command and control, logistics and infrastructure in Syria.

But the group has continued its quest for territory. Gaining control of Kobane would give the Islamic State a hold over a major stretch of the Syrian border with Turkey and open more potential supply lines even as airstrikes target the militants’ financial underpinnings.

The latest clashes have sent refugees streaming toward Turkey, adding to the estimated 1.5 million who have crossed into the neighboring country to escape Syria’s civil war since 2011.

Moustafa Oniedi, a Syrian Kurdish activist based near Kobane, said 10,000 Kurdish fighters have massed to defend the city.

They “are ready to fight until their last breath,” he said by telephone. “Either they die or they win.”

The fighting appeared to intensify Friday after days of seesaw clashes. An activist monitoring Syrian events, Abu Jilan, said at least two airstrikes hit the Kobane area, while Kurdish forces reclaimed several villages from retreating Islamic State fighters.

But the Islamic State fighters regrouped after daybreak and launched a three-prong attack around Kobane, said Oniedi. He claimed the airstrikes did not target front-line Islamic State positions, allowing the militants to quickly resume the siege. The U.S. military has not confirmed any air attacks around Kobane.

Among the latest airstrikes by the U.S.-led coalition were attacks targeting oil sites and Islamic State strongholds in the eastern Deir al-Zour province, according to reports from the Britain-based Syrian Observatory for Human Rights, which monitors the conflict. The area is also an important route connecting Islamic State territory in Syria and Iraq.

Some of the airstrikes in Syria were carried out by warplanes from the United Arab Emirates and Saudi Arabia.

The Arab states are leading opponents of Syrian President Assad. Their participation in the air campaign is likely to be tied to promises from the United States to sharply boost military aid to Syrian rebels seeking to oust Assad in a civil war that has claimed nearly 200,000 lives since 2011.

U.S. Defense Secretary Chuck Hagel said Friday that the United States and its Arab partners have launched 43 airstrikes in Syria and that U.S. planes have carried out more than 200 airstrikes on militant targets in Iraq since the flights were approved by President Obama on Aug. 7.

Collard reported from Beirut. Brian Murphy and Dan Lamothe in Washington contributed to this report.