LONDON (Reuters) - U.S. Defense Secretary Jim Mattis voiced strong support for Turkey’s fight against PKK militants after talks with Turkey’s prime minister on Thursday, as Ankara fumes over a U.S. decision to arm Kurdish fighters in Syria it equates to PKK allies.

U.S. Defence Secretary James Mattis attends the London Somalia Conference' at Lancaster House, May 11, 2017. REUTERS/Jack Hill/Pool

Mattis expressed optimism that Washington and Ankara would work through tensions after a half-hour discussion with Turkish Prime Minister Binali Yildirim in London, which he described as “honest, transparent and helpful.”

It was the highest level talks between the two nations since Washington on Tuesday announced plans to back the YPG militia in an assault to retake the city of Raqqa from Islamic State.

NATO ally Turkey views the YPG as the Syrian extension of the outlawed Kurdistan Workers Party (PKK), which has fought an insurgency in southeast Turkey since 1984 and is considered a terrorist group by the United States, Turkey and Europe.

“We agree 100 percent with Turkey’s concern about PKK ... and we support Turkey in its fight against PKK as a fellow NATO member,” Mattis said.

A U.S. official told Reuters that the United States was looking to boost intelligence cooperation with Turkey to support its fight against the PKK. The Wall Street Journal reported the effort could end up doubling the capacity of an intelligence sharing center in Ankara.

It was unclear if the effort would be enough to soothe Turkey, however.

Turkey has warned the United States that its decision to arm Kurdish forces fighting Islamic State in Syria could end up hurting Washington, and accused its NATO ally of siding with terrorists.

Turkish President Tayyip Erdogan, who will meet U.S. President Donald Trump in Washington next week, has voiced hopes Washington might reverse the decision.

Mattis was upbeat, however, and told reporters traveling with him that Turkey remained committed to the campaign against Islamic State in Syria.

“I have no doubt that Turkey and the United States will work this out with due considerations, significant attention paid to Turkey’s security,” he said.

Yildirim told reporters on Wednesday the U.S. decision “will surely have consequences and will yield a negative result for the U.S. as well”.

The United States regards the YPG as a valuable partner in the fight against Islamic State militants in northern Syria.

Washington says that arming the Kurdish forces is necessary to recapturing Raqqa, Islamic State’s de facto capital in Syria and a hub for planning attacks against the West.

That argument holds little sway with Ankara, which worries that advances by the YPG in northern Syria could inflame the PKK insurgency on Turkish soil.

It has also voiced concern that weapons given to the YPG would end up in the hands of the PKK.

Mattis stated Washington would not give weapons to the PKK.

“We do not ever give weapons to the PKK. We never have and never will,” he said.