Barack Obama has told Turkey that the US was looking at "diplomatic and military" options in Syria, but would not take unilateral action to end the civil war there.
After more than three hours of talks with the Turkish prime minister Recep Tayyip Erdogan at the White House, the US president said both countries would increase pressure on Syria's president, Bashar Al Assad, and offer more support to the opposition.
"We both agree that Assad needs to go," Mr Obama said.
He added the US was looking at a "whole range of options" and that he preserved the "option of additional steps, both diplomatic and military", because the possible use of chemical weapons would also touch US security interests.
But he insisted that the US would not step into Syria unilaterally.
"That is not something that the United States does by itself," Mr Obama said. He said there was "no magic formula for dealing with a extraordinary violent and difficult situation like Syria's".
The US would work to apply steady international pressure on Damascus and to strengthen the opposition, he said.
Mr Obama also mentioned the planned Syria peace conference the US and Russia want to organise.
The meeting will for the first time bring together representatives of the opposition and Mr Al Assad's government.
Last week, Mr Erdogan called for a US-controlled no-fly zone over Syria and said his nation had evidence that the Syrian government had used chemical weapons, evidence he would discuss with Mr Obama.
Mr Erdogan said he wanted to see "the sensitivity of the international community" in the Syria conflict and that it was crucial to have Russia and China join the peace efforts.
While Mr Obama and Mr Erdogan agreed Mr Al Assad must be toppled to end the slaughter, their approaches are different and there are signs of frustration in Ankara at Mr Obama's cautious approach towards the Syrian rebels.
Mr Obama has baulked at providing arms and ammunition to the rebels, fearing they could fall into the hands of extremist elements linked to Al Qaeda, and is now pinning hopes on the peace conference to be jointly organised with Russia.
Turkey, a Nato member an a close partner of the US, shares a border with Syria, where more than two years of fighting between regime forces and rebels has killed more than 70,000 people and displaced millions more, the UN says.
The Washington talks came a day before Russian president Vladimir Putin was to meet the UN secretary general Ban Ki-moon, and a day after UN members voted to condemn an escalation by Syrian government forces.
The meeting came days after 51 people were killed in a twin car bomb attack in the Turkish town of Reyhanli near the Syrian border that the Erdogan government blamed on Syria's intelligence service.
Hakan Fidan, the head of Turkey's national intelligence service MIT and a key figure for Turkish efforts to prove the use of chemical weapons in Syria, was part of Mr Erdogan's delegation to the Washington talks.
* With additional reporting from Agence France-Presse