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.

A Syrian bishop on Friday appealed to the international community to prevent the conflict from reaching Jezireh in northeast Syria, saying it was sheltering 400,000 internally displaced people.

"I am launching an SOS so that you will not be able to say tomorrow with diplomatic hypocrisy: 'We didn't know'," Monsignor Behnan Hindo, the Syrian Catholic archbishop of Hassake-Nisibe, said in an appeal seen by AFP.

Four professional and citizen journalists have been killed in Syria in one week at the hands of regular troops, rebels and Kurdish militia, an international media rights watchdog said on Friday.

State television journalist Basel Tawfiq Yousef was shot dead in near his home in southern Damascus on Wednesday, state news agency SANA said, blaming "armed terrorist groups" for his death, using the regime's term for rebels.

The number of Syrian refugees registered in Lebanon has reached 127,420 and their numbers in neighboring countries have nearly doubled since the beginning of September, the U.N. refugee agency said Friday.

"Across the region... the number of Syrian refugees in surrounding countries now stands at 442,256, an increase of more than 213,000 since the beginning of September," UNHCR spokesman Adrian Edwards told reporters in Geneva.

Turkey's request to NATO for Patriot missiles is "a new act of provocation," Syrian state television quoted a foreign ministry official it did not name as saying on Friday.

Turkey turned to its NATO partners earlier this week to request the deployment of surface-to-air Patriot missiles to protect its troubled border with Syria.

The number of Syrian refugees registered in neighboring countries has nearly doubled since the beginning of September to more than 440,000, the U.N. refugee agency said Friday.

"Across the region... the number of Syrian refugees in surrounding countries now stands at 442,256, an increase of more than 213,000 since the beginning of September," UNHCR spokesman Adrian Edwards told reporters in Geneva.

Turkish Foreign Minister Ahmet Davutoglu said on Friday that Moscow has no reason to worry about the Patriot missiles Ankara has asked NATO to deploy on its troubled border with Syria.

"Patriots are strictly a measure of defense, as Russia and other countries already know," Davutoglu told reporters after a meeting with Ahmed Moaz al-Khatib, the head of the newly-formed Syrian National Coalition.

Kurdish fighters have agreed to join forces in a standoff with hundreds of Islamist rebels in northeastern Syria, an activist opposed to President President Bashar Assad 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.

Iran's influential parliament speaker Ali Larijani met Syrian President Bashar Assad in Damascus on Friday to discuss a solution to the conflict in his country's key ally, reports said.

The meeting came as Moscow warned Turkey against deploying Patriot missiles on its troubled border with war-torn Syria after Ankara turned to NATO to request the surface-to-air defenses.

Russia on Friday warned any deployment of Patriot missiles by Turkey on its border with Syria may create a temptation to use the weapons and spark a "very serious armed conflict" involving NATO.

"I understand that no one has any intention to see NATO get sucked into the Syrian crisis," Russian Foreign Minister Sergei Lavrov told reporters, reiterating concerns aired by the foreign ministry on Thursday.