TURKEY-SYRIA BORDER -- In a phone call Wednesday, President Trump urged Turkish President Recep Tayyip Erdogan to limit an assault on a Kurdish enclave in northern Syria or risk conflict with American forces. The United States, which supports the Kurds, finds itself increasingly at odds with Turkey, a NATO ally.

Kurdish fighters -- America's closest partners in the battle against the Islamic State of Iraq and Syria (ISIS) -- are facing off against Turkish forces, marking a deadly new front in the blood-soaked hills of northern Syria.

The Turkish government is furious with the U.S. for arming the Kurds in Syria, claiming the fighters are controlled by a Kurdish terror organization. Erdogan fears weapons supplied by the U.S. will end up being used against his country.

Turkish President Recep Tayyip Erdogan gestures as he speaks during a joint press conference with Serbia's President Aleksandar Vucic (not in the picture) after their meeting in Belgrade, Serbia, Oct. 10, 2017. REUTERS

On Sunday, he sent tanks across the frontier. Turkey's attacks have so far been limited to the town of Afrin, but the country is threatening to do the same in Manbij, which could bring them close to American personnel.

In Raqqa last year -- then the so-called ISIS capital in Syria -- CBS News witnessed the Kurds fight bravely against the extremists, helping to drive them out of the city.

The U.S. has built bases on Kurdish controlled territory and coordinated airstrikes, but the last straw for Turkey seems to have come this month when the U.S. announced they'd form a 30,000 strong border force.

The U.S. later backpedaled on the border force with Secretary of State Rex Tillerson saying, "Some people misspoke." But that's done little to nullify Turkey.