WASHINGTON (Reuters) - In some of the most critical U.S. comments about Turkey’s advance into northeastern Syria, Defense Secretary Mark Esper told his Turkish counterpart they should deescalate the situation before it becomes “irreparable”, adding that Ankara’s operation could harm U.S. personnel in Syria.

FILE PHOTO: U.S. Defense Secretary Mark Esper holds a news conference with French Defense Minister Florence Parly (not pictured) at the residence of French Defense Minister in Paris, France, September 7, 2019. REUTERS/Christian Hartmann

Turkey stepped up its air and artillery strikes on Kurdish militia in northeast Syria on Friday, escalating an offensive that has drawn warnings of humanitarian catastrophe and turned Republican lawmakers against U.S. President Donald Trump.

The incursion, launched after Trump withdrew U.S. troops who had been fighting alongside Kurdish forces against Islamic State militants, has opened a new front in the eight-year-old Syrian civil war and drawn fierce international criticism.

“While the Secretary reaffirmed (that) we value our strategic bilateral relationship, this incursion risks serious consequences for Turkey,” a Pentagon statement said, giving details of a call between Esper and Turkish Defense Minister Hulusi Akar.

“As part of the call, Secretary Esper strongly encouraged Turkey to discontinue actions in northeastern Syria in order to increase the possibility that the United States, Turkey and our partners could find a common way to de-escalate the situation before it becomes irreparable,” the statement added.

Esper, the statement said, made clear that Turkey’s “uncoordinated actions” risked the progress made in the fight against Islamic State militants.

The call took place on Thursday, the Pentagon statement added.

Turkey says the purpose of its assault is to defeat the Kurdish YPG militia, which it sees as an enemy for its links to insurgents in Turkey. It says it aims to set up a “safe zone” inside Syria, where it can resettle many of the 3.6 million refugees it has been hosting.

The Kurdish YPG is the main fighting element of the Syrian Democratic Forces (SDF) which have acted as the principal allies of the United States in a campaign that recaptured territory held by the Islamic State group.

The SDF now holds most of the territory that once made up Islamic State’s “caliphate” in Syria, and has been keeping thousands of Islamic State fighters in jail and tens of thousands of their family members in camps.