Syria's key National Coalition opposition group will meet May 23 to discuss a U.S.-Russian proposal for an international conference on a political solution to the Syrian conflict.

"The meeting will take place in Istanbul and last three days... to take a decision on the Kerry-Lavrov proposal and our participation," Coalition spokesman Sonir Ahmed told Agence France Presse, referring to U.S. Secretary of State John Kerry and his Russian counterpart Sergei Lavrov.

Israeli Premier Benjamin Netanyahu is "fully determined" to try to halt sales by Russia of advanced missiles to Syria when he visits Moscow for talks with President Vladimir Putin, a minister said Sunday.

A reported Russian plan to sell Damascus S-300 missile batteries "worries us at the highest level and the prime minister is fully determined that this contract should not be fulfilled," Energy and Water Minister Silvan Shalom, a member of the security cabinet, told public radio.

A high-ranking Russian diplomatic source said on Saturday it is was highly unlikely that a proposed international peace conference on Syria could be held this month because of differences over who should take part.

"It is unlikely before the end of May," said the source who participated in talks between Russian President Vladimir Putin and visiting British Prime Minister David Cameron on Friday.

Israeli Prime Minister Benjamin Netanyahu will visit Russia for talks with President Vladimir Putin, officials said Saturday, amid concerns Moscow may be about to deliver advanced missiles to Syria.

Putin's spokesman Dmitry Peskov confirmed the visit to Agence France Presse, but declined to give details, although he told the Interfax news agency it was being planned for next week.

Washington has cozied up to Moscow in search of a political solution in war-torn Syria, but experts aren't sold that there's much common ground between the two.

Amid pressure from a hefty casualty toll and the likely use of chemical weapons by Damascus, the U.S. administration has relaunched its diplomatic efforts with Russia, the powerful protector of President Bashar Assad.

Russia withheld "crucial" information from the United States on one of the suspects of last month's deadly Boston Marathon bombings, The Wall Street Journal reported late Friday.

Citing unnamed U.S. officials, the Journal said on its website that the information consisted of text messages between the mother of Tamerlan Tsarnaev and a Russian relative.

Russia is completing its delivery of surface-to-air missiles to Syria, Russian Foreign Minister Sergei Lavrov said Friday, a move the United States has called destabilizing.

"Russia is not planning to sell -- Russia has sold and signed contracts a long time ago, and is completing supplies of the equipment -- which is anti-aircraft systems, according to the already signed contracts," Lavrov told reporters in Warsaw.

British Prime Minister David Cameron and Russian President Vladimir Putin on Friday discussed joint options for ending the crisis in Syria amid a new diplomatic push to resolve the two-year conflict.

Cameron's rare call on Putin at his summer vacation residence in the Black Sea resort of Sochi came three days after top U.S. and Russian diplomats agreed to make a joint effort in search of a solution.

Lakhdar Brahimi has withdrawn a threat to resign as U.N.-Arab League envoy on the Syria conflict after Russia and the United States launched a joint political initiative, officials said Thursday.

U.N. leader Ban Ki-moon "has asked the joint special envoy to stay on and he has accepted to stay on," U.N. Deputy Secretary General Jan Eliasson told reporters, praising the U.S.-Russia effort to organize a conference on the two-year-old conflict.

Syria on Thursday welcomed a U.S.-Russian initiative to find a political solution to its conflict, counting on ally Moscow to stand firm, even as Washington said President Bashar Assad would have to step down.

U.S. Secretary John Kerry, who announced the initiative earlier this week, said the embattled president would have to step down as part of the resolution to the conflict.