* Kerry: U.S. wants to pressure Assad into talks
* U.S. and other countries exploring options
* War in Syria enters fifth year (Adds State Department comments, paragraphs 2-3, analyst, paragraphs 14-15))
By Lesley Wroughton
SHARM EL-SHEIK, Egypt, March 15 (Reuters) - U.S. Secretary of State John Kerry said the United States would have to negotiate with Syrian President Bashar al-Assad for a political transition in Syria and was exploring ways with other countries to pressure him into agreeing to talks.
But State Department spokeswoman Marie Harf said later that Kerry, in an interview with CBS News that aired on Sunday, was not specifically referring to Assad. She reiterated that Washington would never negotiate with the Syrian leader.
Harf added: "By necessity, there has always been a need for representatives of the Assad regime to be a part of this process. It has never been and would not be Assad who would negotiate - and the Secretary was not saying that today."
Washington has long insisted that Assad must be replaced through a negotiated, political transition, although the rise of a common enemy, hardline militant group Islamic State, appears to have slightly softened the West's stance toward him.
In the CBS interview, Kerry did not repeat the standard U.S. line that Assad had lost all legitimacy and had to go. Syria's civil war is now into its fifth year, with hundreds of thousands killed and millions of Syrians displaced.
"We have to negotiate in the end," Kerry said when asked whether the United States would be willing to negotiate with Assad. "We've always been willing to negotiate in the context of the Geneva I process," he added, referring to a 2012 conference that called for a negotiated transition to end the conflict.
Kerry said the United States and other countries, which he did not name, were exploring ways to reignite the diplomatic process to end the conflict in Syria.
"What we're pushing for is to get him (Assad) to come and do that, and it may require that there be increased pressure on him of various kinds in order to do that," Kerry said.
"We've made it very clear to people that we are looking at increased steps that can help bring about that pressure," he added.
The United States led efforts to convene U.N.-backed peace talks in Geneva last year between Western-backed Syrian opposition representatives and a government delegation. The talks collapsed after two rounds and no fresh talks have been scheduled.
Russia convened some opposition and government figures in January for talks on the crisis but they yielded little progress and were boycotted by the main opposition coalition.
"To get the Assad regime to negotiate, we're going to have to make it clear to him that there is a determination by everybody to seek that political outcome and change his calculation about negotiating," Kerry said.
"That's under way right now. And I am convinced that, with the efforts of our allies and others, there will be increased pressure on Assad."
Andrew Tabler, an expert on Syria at the Washington Institute for Near East Policy, said Kerry's comments did not appear to signal a change in U.S. policy.
"Diplomatic efforts are under way to de-escalate the conflict, which in some cases means negotiating with the regime. But U.S. policy remains that Assad must go as part of a political transition - which isn't going to happen anytime soon," Tabler said.
GOAL OF THE UPRISING
The Western-backed Syrian opposition coalition reiterated that Assad's departure was a demand of the uprising against him.
"The overthrow of the head of the regime and its security apparatus is a key demand of the revolution as part of any future political solution and is also a primary goal of any negotiation process," the National Coalition for Syrian Revolutionary and Opposition Forces said on Twitter on Sunday.
Any attempts to make the revolution change its goal of overthrowing the current government and including it in the dialog would go against the will of the Syrian people, said the opposition group, which despite having only tenuous links with fighters on the ground remains one of the main parties in international discussions to end the war.
Syria sank into civil war after a peaceful street uprising against four decades of Assad family rule began in March 2011. The revolt spiraled into an armed insurgency, which has deepened with the rise of Islamic State and other hardliners.
Assad seems more likely to survive the Syrian crisis than at any point since it began. Iran's support for Assad is as solid as ever, with Russia showing no sign of abandoning him.
U.S-led forces started air strikes against Islamic State group in Syria and Iraq in the summer. Washington has said the campaign in Syria is not coordinated with the Syrian military, which also views the group as its enemy.
(Reporting by Lesley Wroughton in Sharm el-Sheik; Additional reporting by Ali Abdelaty in Cairo and Warren Strobel in Washington; Editing by Sylvia Westall, Rosalind Russell and Peter Cooney)