US Secretary of State John Kerry is hitting the road again this weekend for discussions about the war in Syria in France and Kuwait, just two weeks before peace talks are due to get underway.

In his 22nd trip since becoming secretary of state, Kerry will also push his vision for a Middle East peace treaty, with plans to brief Arab League ministers on progress so far.

Less than a week after returning from Israel, the top US diplomat will leave again on Saturday, heading first for Paris and a meeting of ministers from the "Friends of Syria" grouping, State Department spokeswoman Jen Psaki said Wednesday.

During his stay in the French capital, Kerry will also have discussions with Russian Foreign Minister Sergei Lavrov on Monday.

The two ministers were the prime organizers of Syria peace talks, due to open on January 22 in the Swiss city of Montreux, before political teams then move to Geneva for further UN-led negotiations from January 24.

It is set to be the first time since the war erupted in March 2011 that the Syrian regime of President Bashar al-Assad sits down with the opposition to discuss an end to hostilities.

But while the regime has said it will attend, the fractious, exiled opposition postponed a vote on whether to participate until January 17.

"The debates were very heated among the different groups that make up the coalition. It was not possible to take a decision," a source close to the opposition said, speaking in Istanbul.

US officials have been working behind the scenes to try to persuade the Syrian opposition to attend, saying the peace conference would be "the best opportunity to bring an end to the civil war and the suffering of the people in Syria."

"They're a pivotal part of having a successful conference," Psaki said, saying it would be "significant because it would be the first time that the opposition and the regime are there at a conference together."

But she refused to speculate what would happen if the opposition ultimately refuses to attend the talks.

"We're working with them to put together a delegation," she said. "I'm not going to get ahead of where we are in the process."

Kerry had been expected to return to Israel next week as he seeks to hammer out a framework to guide the negotiations in the coming months.

But Psaki said no decision had yet been made on when the top US diplomat might return for more talks with Israeli and Palestinian leaders.

Instead, he plans to travel to Kuwait for a Syria donor's conference to be held on January 15, which is set to be attended by some 60 countries.

The first such conference in January 2013 led to $1.5 billion in pledges of aid to help the Syrian people of which some 75 percent have honored.