Two main Kurdish groups have agreed to join forces in a standoff with hundreds of Islamist rebels in northeastern Syria, a Syrian Kurdish representative and an activist said on Friday.

Hundreds of fighters loyal to the Kurdish Democratic Union Party (PYD) -- which has close ties to Turkey's rebel Kurdistan Workers' Party (PKK) -- have been locked in fierce battles with fighters of the jihadist Al-Nusra Front and allied Ghuraba al-Sham group in Ras al-Ain on the border with Turkey.

The agreement sets the stage for an expanded conflict in the area between Islamist rebels opposed to Syrian President Bashar al-Assad and Syrian Kurdish forces.

"We initially agreed on forming these (joint) forces that do not belong to any side, and discussions are ongoing now" in Arbil, the capital of Iraq's autonomous Kurdistan region, Mohammed Rasho, a representative of the People's Council of Western Kurdistan, which is close to the PYD, told AFP.

Talks on the formation of the joint forces between the People's Council of Western Kurdistan and the Kurdish National Council, which comprises a number of Syrian Kurdish parties, began three days ago, Rasho said, adding that they took place under the supervision of the presidency of Iraqi Kurdistan.

An activist who identified himself as Havidar meanwhile said that "the two Kurdish national councils in western Kurdistan (Syria) have agreed in Iraq to create a united military force, bringing together PYD forces and other Kurdish dissidents" in Syria.

"Since the Free Syrian Army forces came to Kurdish areas, especially Ras al-Ain," there was in the beginning "an understanding that they would limit their deployment to Arab areas," said Rasho.

But after some time, rebel forces burned Kurdish flags that had been raised, and "clashes between us and them occurred in Kurdish areas," he said.

Rasho added that rebel groups including the Tawhid Brigade, the main opposition formation in Aleppo, Ghuraba al-Sham and "sometimes" the Al-Nusra Front, "stand against Kurdish citizens."

On July 11, the Kurdish National Council met in Iraq with the People's Council of Western Kurdistan and decided to form the Supreme Kurdish Council.

Friday's agreement was announced a day after the Ghuraba al-Sham called in a video posted on the Internet for Islamist volunteers to flock to Ras al-Ain for a drive on the provincial capital Hasakeh, whose population is majority Kurdish.

"We of the Ghuraba al-Sham battalion call on the (mainstream rebel) Free Syrian Army and the mujahedeen to advance towards Ras al-Ain. Increase our numbers so that we can free the city of Hasakeh," an unidentified rebel commander said in the footage, standing among some 50 fighters.

"And we warn all those who stand in the way of this revolt... especially the PYD and the PKK, and any other armed group, against taking any action that contradicts the path of the revolution," he added.

Syria's Arab-led rebels accuse the PYD of being in cahoots with Assad's regime.

Northern and northeastern Syria are home to the majority of the country's two million Kurds.

In July, the army withdrew from majority Kurdish areas, leaving the ethnic group's militia to fend for the minority's safety.

Although Syria's Kurds are opposed to the Assad regime, most have sought to remain neutral in the armed rebellion seeking to topple him.

Over time, they have been dragged into the fighting, after rebel assaults on majority Kurdish areas in key northern provinces.