By Nick Tattersall and Matt Spetalnick
WASHINGTON (Reuters) - President Barack Obama said on Thursday he reserved the right to resort to both diplomatic and military options to pressure Syrian President Bashar al-Assad but insisted that U.S. action alone would not be enough to resolve the Syrian crisis.
Taking a cautious line at a joint news conference with Turkish Prime Minister Tayyip Erdogan, Obama voiced hope that the United States and Russia would succeed in arranging an international peace conference on Syria, despite signs of growing obstacles.
Erdogan had been expected to push Obama, at least in private, for more assertive action on Syria during a visit to Washington this week, days after car bombs tore through a Turkish border town in the deadliest spillover of violence yet.
Obama - who has been reluctant to arm Syrian rebels or become enmeshed militarily in the conflict - made no mention of deeper engagement in Syria during an appearance at the White House, where the leaders sought to project a united front.
"What we have to do is apply steady international pressure," Obama said.
Both leaders stressed the need to bring the Syrian government and opposition to the negotiating table after more than two years of fighting that has killed more than 80,000 people and risks destabilizing the volatile Middle East.
But Russia's insistence on Thursday that Iran, a U.S. foe and Assad supporter, take part in any international talks on Syria could further complicate efforts to organize the meeting.
Russian Foreign Minister Sergei Lavrov said Tehran must have a role in the conference, but that Western states wanted to limit the participants and possibly predetermine the outcome of the talks.
Conflicting comments from Russia and the West over Iran's role in the possible meeting have added to disagreements which already threaten to derail the conference proposed by Moscow and Washington last week.
Erdogan, whose country would be a key player in any conference, suggested that the involvement of Russia and China - both permanent members of the U.N. Security Council - would add impetus, but he made no mention of Iran being invited to attend.
Turkey, a U.S. NATO ally, has been one of Assad's fiercest critics, throwing its weight behind the uprising, allowing the rebels to organize on its soil and sheltering 400,000 refugees.
Earlier on Thursday, Turkish President Abdullah Gul criticized the world's response on Syria as limited to "rhetoric," saying his country had received little help with the refugee influx. Gul's role is largely a ceremonial one.
Turkey has been among the strongest opponents of Assad but its enthusiasm for action against Syria has waned recently, partly in frustration at the fractured Syrian opposition and growing brutality.
Fighters of the al Qaeda-linked Nusra Front in Syria executed 11 men they accused of taking part in massacres by Assad's forces in a video published on Thursday.
A man whose face was covered in a black balaclava shot each man in the back of the head as they kneeled, blindfolded and lined up in a row in the eastern province of Deir al-Zor.
With intelligence assessments that Assad has likely used chemical weapons on a small scale against the opposition, Obama stuck to his position that more specific information is needed to confirm this before deciding how to respond.
Obama, who had said chemical weapons use would cross a "red line," made clear, however, that Washington was keeping all options on the table, though he did not provide specifics.
"There are a whole range of options that the United States is already engaged in," he told reporters. "And I preserve the options of taking additional steps, both diplomatic and military, because those chemical weapons inside of Syria also threaten our security over the long term as well as our allies and friends and neighbors."
But pushing back against the notion that the United States might act alone, Obama said he would present any further chemical weapons evidence to the international community.
"This is also an international problem," Obama said. "It's not going to be something that the United States does by itself, and I don't think anybody in the region including the prime minister would think that U.S. unilateral actions in and of themselves would bring about a better outcome inside of Syria."
With American public opinion running strongly against new military engagement overseas, the White House wants to avoid repeating the mistakes of the Iraq war when false intelligence was used to justify the 2003 U.S.-led invasion.
Erdogan said Turkey, which has been testing blood samples from Syrian casualties, had shared its chemical weapons evidence with the United States, Britain and others and would also take it to the U.N. Security Council at the "proper time."
Underscoring the lack of a Western consensus on how to push Assad from power, Obama declined to set a time frame for the Syrian leader's departure, saying only "the sooner the better."
Erdogan said Turkey was in full consensus with the United States on the need to end the bloodshed in Syria and for a political transition to a government without Assad, but declined to be drawn out on whether Washington should do more.
Erdogan faces growing domestic concern about Turkey's role in Syria and its cost. He said Ankara would maintain its "open-door policy" toward Syrian refugees. He estimated that Ankara had already spent $1.5 billion on the problem.
Touching on another issue of strong U.S. interest, Erdogan said he would go ahead with a planned visit to the Gaza Strip, probably in June, and would also go to the West Bank, despite pressure from Washington to delay the trip.
The Obama administration is concerned Erdogan's visit to the Palestinian enclave might endanger U.S. efforts to revive Turkey's ties with Israel and to advance Middle East peace talks. Erdogan he hoped his visit would promote peace between Israel and the Palestinians.
(Editing by Alistair Bell and Eric Walsh)