The U.S has lost its role as mediator in the Middle East by moving its embassy in Israel to Jerusalem, Turkish President Recep Tayyip Erdogan said.
"With its latest step, America has chosen to be a part of the problem, not a solution, and lost its mediator role in the Middle East peace process," Erdogan told.
Moving the embassy from Tel Aviv to Jerusalem was "very, very unfortunate," he said, warning it would "increase tensions and ignite an even greater fire between communities".
His remarks came as violent clashes erupted along Gaza's border ahead of the controversial embassy opening, leaving 37 Palestinians dead and hundreds wounded in the conflict's bloodiest day in years.
Erdogan urged the international community to "take swift action to put an end to Israel's increasing aggression," denouncing the embassy move as violating international law and UN resolutions.
Jerusalem is one of the most thorny issues of the Israeli-Palestinian conflict, with both sides claiming it as their capital.
Most of the international community does not formally recognise Jerusalem as Israel's capital, and almost all embassies are located in Tel Aviv.
"The establishment of an independent Palestinian state with Jerusalem as its capital is the only way for lasting peace and stability," the Turkish leader said.