WASHINGTON: US President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan vowed to crank up pressure on Syria's President Bashar al-Assad Thursday, but offered no concrete new measures to do so.

Obama warned there was no “magic formula” to force Assad to leave power, as both the United States and Turkey want, but said he hoped a conference that Washington is organizing with Russia next month would be successful.

He gave no sign that he was ready to satisfy Turkish calls for Washington to overcome its reservations about directly arming rebels fighting Assad's regime.

“There is no magic formula for dealing with a extraordinary violent and difficult situation like Syria's,” Obama said, after meeting Erdogan at the White House.

“If there was, I think the Prime Minister and I would already have acted upon it and it would already be finished,” Obama said.

Obama also said that his administration is constantly reviewing his options in Syria, beyond current non-military support for opposition forces and humanitarian aid to refugees.

“I preserve the option of taking additional steps, both diplomatic and military,” Obama said.

Erdogan said US and Turkish goals in Turkey overlapped and said he would continue to discuss how to build a transition in Turkey and to support the opposition in talks later on Thursday with Obama.