U.S. President Donald Trump spoke by phone with his Turkish counterpart Recep Tayyip Erdogan on Wednesday, urging his NATO ally to limit its military actions in northern Syria, according to a White House statement.

The phone call came as intense battles continued between Turkish forces and the U.S.-backed Kurdish troops on the outskirts of the Kurdish-controlled Afrin enclave in northern Syria.

During the talks, Trump urged Turkey to exercise caution and avoid any actions that might risk conflict between Turkish and American forces, according to the statement.

Trump also invited closer bilateral cooperation to address "Turkey's legitimate security concerns."

Turkey on Saturday launched an operation dubbed "Operation Olive Branch" with intense air raids and shelling seeking to oust from the Afrin region the Peoples' Protection Units (YPG), which Ankara considers as a terror group affiliated to the Kurdistan Workers' Party (PKK).

Ankara's military operation followed U.S. officials' statements on a planned Kurdish-led "border security force (BSF)" to be established in the future in Syria along the 900-km border with Turkey, a move that has angered Turkey.

The BSF would be mainly constituted by the YPG, supported politically and militarily by the United States since 2014 in the battle against the Islamic State.

Erdogan said Monday that Turkey will not take a step back from its operation against YPG in Afrin.