US secretary of state John Kerry has said he would be willing to talk to Syrian president Bashar Assad to stem the nation's violence.

In an interview with CBS News, Mr Kerry said the US is pushing for Mr Assad to seriously discuss a transition strategy to quell the Arab country's four-year civil war.

Mr Kerry said: "We have to negotiate in the end."

While previous efforts saw Assad's government not engage on a concrete plan, Mr Kerry said the US is trying "to get him to come and do that".

He said that may require additional pressure on Assad's Iran-backed government, which is fighting Sunni rebels and Islamic State extremists.

Mr Kerry said there may need to be "increased pressure on him of various kinds" to get talks to happen. "We've made it very clear to people that we are looking at increased steps that can help bring about that pressure," he said.

Representatives of the Syrian government took part in talks in Moscow in January with opposition figures, although the main western-backed opposition group shunned the conference.

The nearly four-year conflict has claimed over 200,000 lives, displaced a third of Syria's population, and nurtured the extremist Islamic State group, which now holds a third of both Syria and neighbouring Iraq in its self-declared caliphate.

Mr Kerry said negotiations are important "because everybody agrees there is no military solution; there's only a political solution. But 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. That's under way right now."