By Mert Ozkan and Ellen Francis

HASSA, Turkey/BEIRUT (Reuters) - Turkey opened a new front in Syria's nearly seven-year-old war on Saturday, launching airstrikes against a U.S.-backed Kurdish militia in Afrin province that raise the prospect of a further strain on relations between Ankara and Washington.

The operation, dubbed "Operation Olive Branch" by Ankara, pits Turkey against Kurdish fighters allied to the United States at a time when ties between Turkey and Washington - NATO allies and members of the coalition against Islamic State - appear dangerously close to a breaking point.

The strikes on the Syrian-Kurdish YPG militia hit some 108 targets, the Turkish military said. On land, the Turkey-backed Free Syrian Army rebels were also helping the operation in Afrin, a senior Turkish official said.

"The weakening of the region with artillery fire is under way. The first stage was carried out by aerial forces of the military and nearly all of the targets were destroyed," Prime Minister Binali Yildirim said.

From Sunday land forces would also carry out "necessary activities", depending on developments, he said.

The YPG said the strikes killed six civilians and three fighters. One of the fighters belonged to the YPG and two were from its all-female affiliate, YPG spokesman Birusk Hasaka said. The attacks also wounded 13 civilians, he said.

"We will defeat this aggression, like we have defeated other such assaults," the group said in a statement.

Differences over Syria policy have complicated Turkey's already difficult relationship with NATO ally the United States, which has backed the YPG, seeing it as an effective partner in the fight against Islamic State.

On Saturday, U.S. reaction to the strikes was cautious.

The Pentagon said the United States urged those involved to focus instead on the fight against Islamic State. A Pentagon official said: "We encourage all parties to avoid escalation and to focus on the most important task of defeating ISIS."

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A U.S. State Department official said U.S. Secretary of State Rex Tillerson had spoken to the Russian and Turkish foreign ministers, but gave no details on the calls.

WARPLANES, BUSES

Reuters cameramen in Hassa, near the Syria border, heard the sound of heavy bombardment and saw thick plumes of smoke rising from the Syrian side of the border. The warplanes appeared to be striking from the Turkish side, one of the cameramen said.

Tanks and buses filled with Turkish soldiers and pick-up trucks carrying members of the Turkey-backed Free Syrian Army made their way along a 15-km (nine mile) highway in Turkey towards the border, the cameraman said.

Villages in nearby towns came out to cheer them on, waving Turkish flags. "The best soldiers are our soldiers," some of the villagers shouted in support.

Warplanes pounded parts of Afrin city and villages nearby, while there were skirmishes with Turkish forces and their rebel allies at the edge of Afrin, a YPG official in the area said.

Authorities in the Afrin region say more than a million people live there, many displaced from other parts of Syria.

"Most of the wounded are civilians," said Hevi Mustafa, a top member of the civilian administration that governs Afrin. "There are clashes. There's artillery and shelling. Our units are fiercely responding to this occupation."

The attacks follow weeks of warnings against the YPG in Syria from President Tayyip Erdogan and his ministers. Turkey considers the YPG to be an extension of the outlawed Kurdistan Workers Party (PKK), which has carried out a deadly, three-decade insurgency in Turkey's mainly Kurdish southeast.

Turkey has been particularly outraged by an announcement that the United States planned to train 30,000 personnel in parts of northeast Syria under the control of the YPG-spearheaded Syrian Democratic Forces.

"In a situation like this our expectations from everyone and especially from our allies is that they side with us, not with terrorists," Turkish Foreign Minister Mevlut Cavusoglu said, appearing to refer to Washington.

The attacks could also complicate Turkey's push to improve its relationship with Russia. Moscow, the main backer of Syrian President Bashar al-Assad, will demand in the United Nations that Turkey halt the operation, RIA news reported.

The Syrian government, which has threatened to shoot down Turkish planes, condemned the strikes. It called the attacks "brutal Turkish aggression on Afrin which is an intrinsic part of Syrian land", according to state media.

GROWING STRENGTH

The YPG's growing strength across large parts of northern Syria has alarmed Ankara, which fears the creation of an independent Kurdish state on its southern border. Syrian Kurdish leaders say they seek autonomy as part of Syria, not secession.

Turkish officials have said the operation is likely to continue toward Manbij, further east. They also said that thousands of pro-Turkey civilians had escaped the YPG-controlled areas in an attempt to reach Aleppo.

But the Syrian Observatory for Human Rights, a U.K-based monitoring organization, said it was not true that people were fleeing en masse.

The Pentagon spokesman said the United States recognised Turkey's security concerns about the PKK, noting it was designed by Washington as a foreign terrorist organization.

"We will destroy the terror corridor gradually as we did in Jarabulus and Al-Bab operations, starting from the west," Turkey's Erdogan said, referring to previous operations in Syria to push out Islamic State and check the YPG's advance.

Earlier on Saturday, the military said it hit shelters and hideouts used by the YPG and other Kurdish fighters, saying Kurdish militants had fired on Turkish positions inside Turkey.

But the U.S.-backed Syrian Democratic Forces - which the YPG spearheads - accused Turkey of using cross-border shelling as a false pretext to launch its offensive in Syria.

(Additional reporting by Osman Orsal in Hassa; Orhan Coskun, Tulay Karadeniz, Gulsen Solaker and Tuvan Gumrukcu in Ankara; Omer Berberoglu, Ezgi Erkoyun in Istanbul; David Brunnstrom and Arshad Mohammed in Washington; Writing by David Dolan, Editing by William Maclean)