A mortar round hit the Russian embassy compound in Damascus on Sunday, injuring three, as Moscow accused Washington of blackmailing it to win a tough UN resolution on Syria's chemical weapons.
The United States condemned the shelling, which came as clashes in the northwestern province of Idlib claimed the life of a local leader of the Al-Qaeda-affiliated Islamic State of Iraq and the Levant (ISIS).
"On September 22 as a result of shelling by the rebels of the Damascus neighbourhood of Mazzeh, one of the shells exploded on the territory of the Russian Embassy in Syria," the Russian foreign ministry said.
"Three employees received non-life threatening injuries. Right now an investigation of the incident is under way."
The ministry added that the embassy was considering additional security measures after the attack, the first time the compound has been hit.
US State Department deputy spokeswoman Marie Harf said Washington condemned the shelling and "expresses its concern for the welfare of those injured in the incident".
But in Moscow, Russia's Foreign Minister Sergei Lavrov lashed out at the United States, accusing Washington of using "blackmail" to push for a tough UN resolution enshrining the deal under which Syria is turning over its chemical weapons for destruction.
The United States, Britain and France want a severe resolution that could include sanctions or use of force under the UN Charter's Chapter VII if Syria fails to implement the deal.
"Our American partners are beginning to blackmail us: if Russia won't support a resolution under Chapter VII in the UN Security Council, then we will stop the work in the Organisation for Prohibition of Chemical Weapons," Lavrov told Channel One, Russian agencies said.
The Hague-based OPCW is charged with overseeing the destruction of Syria's weapons arsenal.
"Our partners are now blinded by their ideological goal of regime change (in Syria)," Lavrov said.
"All they talk about is that (President) Bashar al-Assad must leave.
"They are only interested in proving their own superiority. Not in the goal that is guiding us, to solve the problem of chemical weapons in Syria," he added.
The OPCW said Saturday it had received a complete inventory from Syria of its chemical arsenal and was scrutinising the data.
CNN later quoted a senior US administration official as saying that US officials were surprised and encouraged by the volume of information provided by Damascus.
"The official said the declaration was more complete than what the officials had expected the Syrians to put forth," CNN reported.
A tentative Saturday deadline had been set for the full accounting of Syria's arsenal under the plan which calls for its chemical weapons to be destroyed by mid-2014.
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There are hopes that the deal could pave the way for peace talks to end the 30-month conflict which has killed more than 110,000 people and forced two million more to flee abroad.
But the plan has been greeted with scepticism by the Syrian opposition.
The OPCW, which enforces the Chemical Weapons Convention that Syria has applied to join, postponed a meeting of its Executive Council due Sunday to discuss the practicalities of disposing of Syria's chemical weapons.
The deal was worked out after Washington threatened military action in response to an August 21 chemical weapons attack outside Damascus.
Most Western governments accuse forces loyal to Assad of unleashing the chemical weapons that killed hundreds in suburbs of the capital.
A UN report released last Monday said that sarin gas was used in the Syria attacks but did not apportion blame. Assad's government and the rebels fighting to oust him have accused each other of being responsible.
The Syrian conflict is expected to take centre stage at UN headquarters when the annual General Assembly opens on Tuesday.
President Hassan Rowhani of Iran, a leading Syrian ally, is among the keynote speakers and has offered to mediate between the opposition and the Damascus government.
But the opposition Syrian National Coalition has dismissed the offer as "not serious".
A delegation from the Coalition has arrived in New York ahead of the General Assembly meeting, a spokeswoman for the grouping said on Sunday.
Delegates include Coalition president Ahmad Jarba and prominent members Michel Kilo and Burhan Ghalioun.
The spokeswoman declined to give details of the schedule for the trip, which lasts until October 1, but said it would include meetings with international representatives.
Turkey, meanwhile, said it would not "tolerate" Al-Qaeda-affiliated groups emerging in the Syrian conflict, after clashes between the radical groups and mainstream rebels this week.
"We will not allow any formation beyond our borders that would threaten Turkey and the entire region," President Abdullah Gul was quoted as saying by the Milliyet newspaper on Sunday.
"We can never tolerate this."
On Sunday, Abu Abdullah al-Libi, a local chief of the Al-Qaeda-affiliated ISIS, was killed in clashes with other rebels in Idlib province.
He was killed along with 12 other fighters from the jihadist organisation, the Syrian Observatory for Human Rights said.