Syrian Kurds battled to defend a key border town from an Islamic State advance on Monday as Kurdish youths from neighboring Turkey rushed to their aid, heightening the pressure on Ankara to act against the Islamist insurgents.

In Turkey, which is struggling to manage an influx of more than 130,000 Syrian Kurdish refugees since Friday, security forces fired tear gas and water cannon at hundreds of Kurdish protesters who accuse Ankara of favoring Islamic State against the Kurds.

The main Kurdish armed group in northern Syria, the YPG, said its fighters had halted the Islamic State advance east of the predominantly Kurdish town of Kobani, also known as Ayn al-Arab, but that fierce fighting was continuing.

Hundreds of Kurdish youths gathered on the Turkish side of the border, responding to calls from Kurdish leaders to join the fight against Islamic State fighters who have seized swathes of Iraq and Syria and proclaimed a caliphate.

Residents fleeing Kobani said the militants were executing people of all ages in villages they seized. Kurds are mostly Sunni Muslims but Islamic State views them as apostates because of their secular ideology. It has persecuted and killed Shi'ite Muslims, Christians and members of the ancient Yazidi sect as well as moderate Sunnis who reject its stark version of Islam.

Turkish security forces are now trying to keep Kurds from crossing the frontier to aid their brethren. At the Mursitpinar border crossing, a line of paramilitary police stood guard along a barbed-wire border fence.

"We all want to cross the border. We tried yesterday but they attacked us, and we will try again today," said balaclava-clad Kurdish activist Shirwan, 28, holding a large PKK flag.

Ismet, 19, a local man who makes a living collecting strawberries, said the protesters had gathered from cities across Turkey's predominantly Kurdish southeast: "They are not from around here. They come from Sirnak, Van, Mardin, Nusaybin."

He said several hundred Turkish Kurds had already crossed to join the fight. Other residents put the figure higher.

The advances by the Sunni insurgents just across Turkey's southern border have alarmed Ankara. But so far Turkey has been slow to join calls for a coalition to fight Islamic State, worried in part about links between the Syrian Kurds and Turkey's own Kurdistan Workers' Party (PKK), which waged an armed campaign for Kurdish rights over several decades.

Turkey strongly denies it has given any form of support to the Islamist militants, but Western countries say its open borders during Syria's three year civil war allowed Islamic State and other radical groups to grow in power.

The PKK called Turkey's Kurds to arms on Sunday, saying "supporting this heroic resistance" in Kobani was a "debt of honor". Radio stations played patriotic Kurdish songs about heroic fighters and martyrs and one played recordings of PKK commander Murat Karayilan in a bid to drum up support.

Deputy Prime Minister Numan Kurtulmus said more than 130,000 Syrian Kurds had fled to Turkey since Friday.

TREADING DIFFICULT LINE

Fierce clashes continued on Monday but the Islamic State advance to the east of Kobani, scene of the fiercest fighting since the insurgents launched their offensive last Tuesday, had been halted, Redur Xelil, spokesman for the YPG, said via Skype.

He said hundreds of Turkish Kurds were already helping in the struggle to push back the insurgents.

The Syrian Observatory for Human Rights, which tracks violence in the Syrian war, said Islamic State fighters had made no significant advance in the last 24 hours.

The United States has carried out air strikes against Islamic State fighters in Iraq and says it is prepared to extend them into Syria, but has not said when or where strikes would begin.

Turkey, a NATO member with a big U.S. air base in the southern town of Incirlik, has so far made clear it does not want to take a frontline military role. Initially, officials said they were wary of fighting Islamic State because the group was holding 46 Turkish hostages.

Those hostages - including Turkey's consul general in the Iraqi city of Mosul, soldiers and children - were freed on Saturday, but Turkish officials said policy toward Islamic State was unlikely to change.

"The hostages weren't the only concern for our Iraq and Syria policy," said one senior Turkish official, declining to be identified so as to speak more freely.

"There are security problems especially in the Kurdish regions of Syria. We are always ready to help them but that doesn't mean that we will carry out a military operation ... Turkey will continue to be a part of the coalition but our policies on Iraq and Syria will not change," he said. 

Turkey launched a peace process in 2012 to end a 30-year-old conflict with the PKK, listed as a terrorist organization by Ankara, the European Union and the United States. Mutual suspicion still runs high between Kurdish communities and the security forces in large parts of the southeast.

In Iraq, where there is an autonomous Kurdish region with its own official security forces, European countries, including Germany, France and Italy, have already agreed to send the Kurds light weapons to use against militants.

But in Syria, where Kurdish fighters have no official status and their leaders are more closely linked to the banned PKK, outside help is more complicated.

Opposition politicians in Germany have called for the PKK to be taken off the EU list of terrorist groups.

Turkey remains strongly opposed to such a move and diplomats say it would not be considered without Turkish accord.

"The threat to the Yazidis and Christians in northern Iraq is no reason, in my view, to reconsider the ban," German Foreign Minister Frank-Walter Steinmeier told broadcaster ARD on Sunday.

"The ban stands. We are delivering weapons to (Iraqi) Kurdish security forces; that is what we decided."

(Additional reporting by Tom Perry in Beirut, Alexandra Hudson in Berlin and Orhan Koskun in Ankara; Writing by Nick Tattersall; Editing by Peter Graff)