Washington will be invited to fresh Syria peace talks being organised by Moscow and Ankara this month, Turkey's foreign minister said, but Russia declined to confirm the invitation on Friday.
On the ground meanwhile, Syria accused Israel of bombing a key airbase near the capital Damascus before dawn, condemning the incident as a "desperate attempt to support terrorist organisations."
Despite backing opposite sides in the Syrian conflict, Russia and Turkey have worked closely in recent weeks to broker a nationwide ceasefire that is meant to pave the way for January 23 peace talks in the Kazakh capital Astana.
In the past, Washington has played a key role in attempts to bring Syria's warring parties to the negotiating table, but it has been notably absent from the cooperation between Ankara and Moscow.
Turkish Foreign Minister Mevlut Cavusoglu, whose country, like Washington, backs Syria's rebels, nonetheless insisted Thursday that US officials would be invited.
"The United States should be definitely invited, and that is what we agreed with Russia," he said.
"Nobody can ignore the role of the United States. And this is a principled position of Turkey," he added.
But the Kremlin, a key ally of Syrian President Bashar al-Assad's government, declined to comment on Cavusoglu's statements.
"I cannot say anything about this for now," Kremlin spokesman Dmitry Peskov said.
He added however that Russia is "interested in the broadest possible representation of the parties who have a bearing on the prospects of a political settlement in Syria."
The Russian foreign ministry later issued a statement to say there had been "tripartite discussions" between Russia, Iran and Turkey on the holding of the talks.
- US to take back seat -
Last month, Russian Foreign Minister Sergei Lavrov expressed hope that the next American administration would "join the efforts so that we can work in the same direction harmoniously and collectively."
The Astana talks are scheduled to begin just three days after president-elect Donald Trump is inaugurated.
In recent months, Washington has been largely absent from international discussions about Syria, and experts say Trump is unlikely to focus on the conflict.
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"For the new American administration, it's not a priority to play a role in resolving the Syrian crisis," said Imad Salamey, head of the political science department at the Lebanese American University.
"I think that with Trump in the White House, leadership will stay in the hands of Russia, but they will demand guarantees, particularly on Israel's security and the reduction of Iran's role," he told AFP.
Invitations to the talks have yet to be sent out, and the format of the discussions remains unclear.
A source close to the Syrian government said it expected the meeting to open with a session including all the invited parties but most of the discussions would take place directly between the government and rebels under Russian and Turkish supervision.
- Syria accuses Israel of strikes -
Ankara and Moscow laid the groundwork for the talks with a nationwide truce that began on December 30 and has brought quiet to large parts of the country.
Fighting has continued, however, in the Wadi Barada region outside the capital, which is the main water source for Damascus.
Supply from the area has been cut since December 22 after clashes damaged infrastructure, leaving 5.5 million people in Damascus and its suburbs without water.
The government has vowed to retake the area, and was advancing on the ground on Friday, the Syrian Observatory for Human Rights said.
The Syrian army meanwhile accused Israel of carrying out missile strikes on the Mazzeh airbase outside the capital early on Friday.
"In a desperate attempt to support terrorist organisations, Israeli enemy aircraft launched missiles from the north of Lake Tiberias (the Sea of Galilee) at 00:25 am (2225 GMT Thursday)," a military source told the state SANA news agency.
"The Syrian armed forces warns the Israeli enemy of the repercussions of this blatant aggression, and insists on continuing the war on terrorism to eliminate it," the source added.
The Israeli army had no comment on the strikes when contacted by AFP earlier on Friday.
Syria has accused Israel of carrying out several strikes on its territory and the two countries technically remain at war.
More than 310,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.