The US is prepared negotiate with Syria's president Assad in order to end to the country's brutal civil war, which has moved into its fifth year, US Secretary of State John Kerry said.

Washington has long insisted that Assad must step down in order for groups battling for control of the country since the outbreak of the civil war in 2011 to reach a peaceful settlement.

However, in the wake of the rise of a common enemy in Islamic State (Isis), Kerry has indicated that the US may be prepared to soften its stance towards Assad.

More than 215,000 people have been killed in the conflict, which Kerry described as "one of the worst tragedies any of us have seen".

"We have to negotiate in the end," Kerry said. "We've always been willing to negotiate in the context of the Geneva 1 process," he added, referring to a 2012 conference in which the outlines of a negotiated settlement were discussed.

He said that the US and other countries were attempting to restart the negotiation process.

"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," he said in an interview with CBS News.

"We've made it very clear to people that we are looking at increased steps that can help bring about that pressure," he added.

A recent round of peace talks convened in Geneva in 2014 collapsed in their initial stages, and another round convened by Assad's backer Russia also failed to deliver an agreement between the parties involved, after they were boycotted by key rebel groups.

Kerry said that increased international pressure was required in order to get Assad to return to the negotiating table.

"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."

US president Barack Obama was close to authorising air strikes against Syrian government forces in 2013, after an alleged gas attack on civilians in Damascus.

But having retained the backing of Russia and Iran, Assad's grip on power seems increasingly secure, as the US led coalition focusses its efforts on its campaign to "degrade and destroy" Isis.