The Turkish army has shelled positions held by Kurdish-backed militia in northern Syria for a second day, state media confirms.

Key points: Turkey shells Kurds in Syria for second day

Turkey shells Kurds in Syria for second day Syria condemns the attack, urges UN to step in

Syria condemns the attack, urges UN to step in Turkey views the Kurdish YPG as an extension of the PKK, Washington does not

Turkey views the Kurdish YPG as an extension of the PKK, Washington does not Pro-Kurdish protests in Istanbul turn violent as demonstrators clash with police

On Saturday, Turkey demanded the Syrian Kurdish YPG militia withdraw from areas that it had captured in northern Aleppo in recent days from insurgents in Syria, including the Menagh air base.

The shelling has targeted those areas.

Syria's Government condemned the Turkish offensive on Sunday, urging the United Nations to intervene.

"The foreign ministry strongly condemns the repeated Turkish crimes and attacks against the Syrian people and Syria's territorial integrity," state news agency SANA said.

The ministry called on the United Nations Security Council to "put an end to the crimes of the Turkish regime".

Turkey has been alarmed by the expansion of Kurdish sway in northern Syria since the start of the conflict in 2011.

The ministry statement said that on Saturday, 12 pick-up trucks equipped with heavy machine guns and ammunition had crossed into Syria from Turkey via Bab al-Salama.

They "were accompanied by around 100 gunmen, some of them Turkish forces and Turkish mercenaries," SANA added.

The French Foreign Ministry has called on Turkey to halt the shelling, saying it was worried about the "deteriorating situation" in the region.

We call for the cessation of all bombardments, those of the regime and its allies on the entire territory and those of Turkey in the Kurdish zones," a statement said.

It added the priority should be the fight against Islamic State and application of agreements reached by the major powers in Munich on Friday.

But Prime Minister Ahmet Davtoglu reportedly told German Chancellor Angela Merkel his security forces would continue to respond to attacks by the Kurdish militia.

Pro-Kurdish protests in Istanbul turn violent

Pro-Kurdish media said police used live ammunition during clashes with protestors in Istanbul. ( AFP: Yasin Akgul )

Meanwhile Turkish police clashed with pro-Kurdish protesters in a flashpoint district of Istanbul, leaving at least two wounded, reports said.

Protesters in the Gazi district of the European side of the city — known as a staunchly anti-government stronghold — came out to protest against the military operations.

Police arrived in armoured vehicles and fired tear gas canisters, while wounded protesters were taken away on stretchers, witnesses said.

Pro-Kurdish media reports said at least two were wounded and the police had used live ammunition, but this was not confirmed by official sources.

Washington, Ankara disagree over Kurds

The YPG controls nearly all of Syria's northern frontier with Turkey, and has been a close ally of the United States in the campaign against the Islamic State militant group in Syria.

But Ankara views the group as an extension of the Kurdistan Workers Party (PKK), which has waged a three-decade-old insurgency for autonomy in south-east Turkey.

Prime Minister Ahmet Davutoglu said on Saturday the shelling had taken place under "the rules of engagement against forces that represented a threat in Azaz and the surrounding area".

He demanded the Menagh base be evacuated and said he had spoken to US Vice President Joe Biden to make that point and stress that the YPG, the military arm of the PYD, was an extension of the PKK and a direct threat to Turkey.

The shelling intensified at 2:00am (local time) before dying down but not stopping, according to the Syrian Observatory for Human Rights monitoring group, which reports on the war using a network of sources on the ground.

Washington has been relying on the YPG as one of its few effective allies on the ground in the fight against the Islamic State group.

The PKK is recognised by the United States as a terror group but not the YPG or PYD.

The issue is causing increasing friction between the two NATO allies and complicating efforts to find a solution to Syria's almost five-year civil war.

US State Department spokesman John Kirby said in a statement late on Saturday that Washington urged Turkey to cease its cross-border artillery fire.

He said the United States had also urged the Kurdish fighters "not to take advantage of a confused situation by seizing new territory".

Syria's military, backed by Russian air strikes, is fighting Syrian insurgents in the same area, trying to seal the frontier with Turkey and reclaim areas of Aleppo city held by rebels.

Syrian rebels say the YPG is fighting with the Syrian military and its allies against them in the five-year-old civil war.

The YPG denies this.

On Friday, US and Russian ministers meeting in Munich agreed to implement a ceasefire within a week.

Reuters/AFP