The United States will have to negotiate with Syrian President Bashar al-Assad to end the civil war, which is now entering its fifth year, U.S. Secretary of State John Kerry told CBS News in an interview on Sunday.

Washington has long insisted that Assad must go, but the rise of a common enemy, the hardline militant group Islamic State of Iraq and al-Sham (ISIS), appears to have slightly softened the West's stance towards him.

Emphasizing that the conflict was one of the worst tragedies the world has seen in which more than 215,000 people are estimated to have been killed, Mr. Kerry said the U.S. was exploring ways to pressure Assad into agreeing to talks after two previous rounds of talks collapsed.

"We have to negotiate in the end," Kerry said. "We've always been willing to negotiate in the context of the Geneva I process," he added, referring to a 2012 conference which called for a negotiated transition to end the conflict.

The United States led efforts to convene a 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.

"To get the Assad regime to negotiate, we're going to have to make it clear to him that there is 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."

Syria sank into the 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 ISIS 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 ISIS 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, according to Reuters.

The war has killed more than 200,000 people and displaced close to half the population, according to the United Nations. Damascus accuses its Western and Gulf Arab opponents of seeking to destroy the country by providing aid to an insurgency now dominated by jihadists, who also pose a threat to the West.