John Kerry said the United States has been working behind the scenes with a number of "critical players" to resolve the Syria crisis diplomatically. (Andrew Harrer/Bloomberg News)

Secretary of State John Kerry said the United States and its allies are significantly increasing diplomatic efforts to pressure Syrian President Bashar al-Assad on ending the civil war in his country and transitioning out of power.

In an interview with CBS News that aired Sunday, Kerry said the United States has been working behind the scenes with "a number of different critical players in this tragedy" to find a diplomatic outcome.

Kerry said efforts are underway to pressure the Assad regime to agree to talks.

“We have to negotiate in the end,” he said. “And 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. We’ve made it very clear to people that we are looking at increased steps that can help bring about that pressure.”

President Obama first called on Assad to leave office in August 2011, after the Syrian leader used violence against protesters inspired by Arab Spring revolutions.

But as the Syrian civil war has dragged on, Western diplomats — and some in the Obama White House — reportedly have concluded that Assad is unlikely to be ousted militarily anytime soon. In addition, one of Assad’s enemies in that war, the Islamic State militant group, has become an international concern in its own right. If Assad ever did step down, Western governments worry, an even more dangerous and brutal regime might fill the vacuum in Syria.

Now, the United States continues to insist that Assad should still leave power, but — now that Assad is the enemy of America’s enemy — it has seemed to soften its demands for how and when he should go. The United States has publicly supported international negotiations aimed at slowing down the Syrian civil war, with Assad’s government a key participant in the talks.

In recent months, Kerry has refrained from making any explicit demands that Assad leave office. Kerry previously backed efforts by the United Nations to broker some local cease-fires and Russia's attempts to host peace talks between the Syrian government and the opposition.

"Everybody agrees there is no military solution. There's only a political solution," Kerry said. "I am convinced that, with the efforts of our allies and others, there will be increased pressure on Assad."

Carol Morello contributed to this report.

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