The launch came despite warnings that the operation could be militarily tough against an already battle-hardened foe and complicate relations with both Washington and Moscow.

Turkey's army said operation "Olive Branch" began at 1400 GMT and was aimed at the YPG and Islamic State (IS) jihadists.

- Huge plumes of smoke -

An AFP correspondent on the Turkish side of the border saw two war planes launch air strikes inside Syrian territory, sending huge white plumes of smoke into the sky.

Erdogan said Turkish forces would next seek to oust the YPG from Manbij, a town further east.

In a delicate diplomatic situation, the top diplomats of Russia, Iran and the United States in Ankara were invited to the foreign ministry to receive a briefing on the operation, the ministry said.

Foreign Minister Mevlut Cavusoglu held telephone talks with US counterpart Rex Tillerson while Turkey's top general Hulusi Akar informed his American and Russian counterparts.

Turkey accuses the YPG of being the Syrian offshoot of the Kurdistan Workers' Party (PKK) which has waged a rebellion in the Turkish southeast for more than three decades and is regarded as a terror group by Ankara and its Western allies.

But the YPG has been the key ally of Turkey's fellow NATO member the United States in the fight against IS jihadists, playing a key role in pushing the extremists out of their Syrian strongholds.

A senior US State Department official said on Friday that Washington did not believe "a military operation... serves the cause of regional stability."

Erdogan had reacted furiously this week to an announcement of plans to create a US-backed 30,000-strong border security force in northern Syria composed partly of YPG fighters, describing it as an "army of terror".

Tillerson later said the "entire situation has been mis-portrayed, mis-described", admitting "we owe them (Turkey) an explanation."

"We don't care what they say," Erdogan spat back. "They will learn how wrong it is to trust a terror organisation."

- 'Russian green light?' -

The Syrian foreign ministry however strongly denied this, denouncing the operation as a "brutal Turkish aggression".

Turkey from August 2016 to March 2017 pushed into Syria in its more than half-year Euphrates Shield operation in an area to the east of Afrin against both YPG and IS.

Analysts say that crucial for any major new ground operation will be approval from Moscow which has a military presence in the area and a cordial relationship with the YPG.

Russia is an ally of the Assad regime which Turkey has opposed since the onset of the war. But both Ankara and Moscow, as well as Tehran, have worked closely on a peace process in the last year.

"Moscow is concerned at this news. We call on the opposing parties to show restraint," the Russian foreign ministry said.

But the Russian defence ministry said its troops were withdrawing from the Afrin area to prevent any "provocation" and ensure the security of its troops.

Timur Akhmetov, Ankara-based researcher at the Russian International Affairs Council, told AFP that Russia appeared to have given the "green light" to the operation but made clear it should not lead to destabilisation elsewhere.

"I don't think Russia will agree to let Turkey occupy the whole Afrin region and insists on keeping the Syrian government in charge," he added.

The Syrian Democratic Forces (SDF), an umbrella grouping composed mainly of YPG, said in a statement the Turkish operation threatened to "breathe new life" into IS and said it has "no choice but to defend ourselves and our people".