Countries supporting the Syrian uprising against President Bashar al-Assad meet on Wednesday in Amman to discuss ways to end the bloody conflict, mainly a US-Russian proposal for peace talks.

Eleven top diplomats from Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and the United States, are attending the meeting of the Friends of Syria group.

"This meeting ... in Amman is to bring together all of the key players in the region as well as the key partners in Europe and the United States to talk about strategy," a senior US State Department official said.

"It's basically to review where we are on Syria."

Ahead of the meeting, US Secretary of State John Kerry will hold a news conference at around 1130 GMT, after British Foreign Secretary William Hague meets with reporters at 0700 GMT.

Syrian Ambassador to Jordan Bahjat Suleiman will also hold a rare press conference around 0900 GMT.

Earlier this month the United States and Russia, which support opposite sides in the Syrian conflict, proposed a peace conference, dubbed Geneva 2, which would bring together rebels and representatives of Assad's regime.

The US official described the conference as "the most serious effort in the last two years to get the Syrian government to sit down and negotiate with the Syrian opposition...

"In my experience we haven't seen a push that has such clear support from the Russians, from the United Nations," he said, speaking by phone from Amman with reporters accompanying Kerry while in Oman.

A French diplomatic source said the Geneva conference "comes at a time when the regime is taking the lead on the ground as its supporters are getting more and more involved."

Jordan has announced that for the first time in the 26-month conflict, the opposition, whose chief Ahmed Moaz al-Khatib resigned last month, has not been invited to the Friends of Syria meeting.

The main opposition National Coalition member Anas Abdeh told AFP on Monday that the opposition had not been invited "because the Friends of Syria want to reach a unified position on the international conference."

Their common stance "will be revealed at the opposition's meeting in Istanbul (Thursday), where they (Friends of Syria) will try and convince the opposition to take part in the (international) conference," Abdeh added.

In Turkey, the opposition is set to choose a new leader and to decide whether it will take part in Geneva 2.

"One of the things we'll be talking about here in Amman... is what else needs to be done with respect to the military balance on the ground," the US official said, urging the opposition to unify ranks.

"The first thing to do is for the Syrian opposition coalition -- which we and Friends of Syria have all recognised as the legitimate representative of the Syrian people -- we need that opposition coalition to resolve its leadership questions and then turn to address its stance on Geneva. First things, first," he said.