At least 300 Kurdish fighters crossed from Turkey into Syria overnight to battle jihadists from the Islamic State group trying to seize a strategic border town, a monitor said Saturday.
The reinforcements arrived as Kurdish officials and the Syrian opposition urged the international community to prevent the "ethnic cleansing" of the strategic town of Ain al-Arab, known by the Kurds as Kobane.
The Syrian Observatory for Human Rights said the Kurdish fighters had joined the Syrian Kurdish People's Protection Units (YPG) in their fight against IS.
The jihadist organisation has been fighting since Tuesday night to try to capture Ain al-Arab, and has seized 60 villages in the area.
The fighting has prompted a mass exodus into Turkey, which opened its border on Friday to fleeing Kurds and has since received 45,000 people, according to officials.
At the same time, the fate of at least 800 residents of the captured villages remains "unknown," the Observatory says.
The clashes prompted Syria's opposition National Coalition to urge air strikes to "stop mass atrocities" if IS advances into Ain al-Arab.
"Air strikes are needed to help opposition forces protect vulnerable civilians," the coalition's US representative Najib Ghadbian said.
Key Syrian Kurdish official Salih Muslim Mohamed, a leader of the Democratic Union Party (PYD), launched his own appeal for aid.
He urged the United States and Europe to help Ain al-Arab avoid the fate of the Iraqi town of Sinjar, which has been emptied of its Yazidi minority residents in the wake of an IS onslaught.
"Kobane is facing the most barbaric attack in its history," he warned.
"If you want to avoid an ethnic cleansing even more barbaric than that in Sinjar, you must support Kobane because the next few hours will be decisive," he added in a statement.
"Kobane is waiting for your urgent action."
The Observatory also reported Saturday that IS had executed at least 11 Kurds, among them two teenagers, in the area around the town.
Ain al-Arab is Syria's third-largest Kurdish town, and capturing it would give IS control of a long stretch of the country's northern border with Turkey.
The Observatory said fighting was continuing on Saturday, with heavy clashes in the vicinity of the town.