The U.S. government called Saturday on Turkey to stop shelling American-backed Kurdish fighters in northern Syria as the militants sought to seize new ground before a possible cease-fire, creating dangerous fissures between tenuous allies in the war against Islamic State extremists.

Feb. 4, 2016: Kurds demonstrate outside the conference center in London.

The U.S. State Department and the Pentagon both pressed Turkey to immediately stop shelling and urged America’s Kurdish allies in Syria not to expand their areas of control as world leaders struggle to cement the details of a cease-fire meant to take hold within days.

The surge in violence threatens to drive a new wedge between the U.S. and Turkey, wary allies in the war against Islamic State. And it is a reflection of the fractured Syrian battlefield that makes it difficult for world leaders to work out a durable cease-fire in the five-year-old war.

American officials stepped in to try to quickly bring an end to violence that erupted after Turkey followed through on its vow to attack the Kurdish rebels in northern Syria that it views as a threat.

In identical statements, the Pentagon and the State Department called on Turkey and Kurdish militants to take steps to prevent the violence from getting worse.

“We are concerned about the situation north of Aleppo and are working to de-escalate tensions on all sides,” the State Department and the Pentagon said in their statements.

The U.S. appeal came after Turkish Prime Minister Ahmet Davutoglu demanded Kurdish fighters withdraw from a one-time Syrian military base near the town of Azaz, a move that had brought the Kurds closer to Turkey’s border.

The U.S. military is working closely with Kurdish forces in northern Syria, where they have delivered some of the most decisive and lasting blows to Islamic State extremists. But Turkey considers the most effective Syrian Kurdish force, known as the YPG, to be terrorists just like the PKK, the Kurdish insurgent separatists classified as a terrorist group by the U.S., Turkey and the European Union.

As U.S. cooperation with the YPG has deepened in Syria, the PKK has embarked on a new fight with Turkish security forces in southeastern Turkey towns and cities close to the Syrian border.

Turkey has provided the U.S. with evidence it says shows that the YPG has smuggled large amounts of firepower, including weapons made in America, to PKK fighters in Turkey, according to officials from both countries.

U.S. officials said they have looked into each case and found no evidence that any arms or ammunition it has given directly to Syrian Kurdish forces to fight Islamic State have been smuggled into Turkey to be used against Turkish security forces.