Russia has said that a shell landed on the grounds of its embassy compound in Damascus, without inflicting any casualties.
The mortar landed on the embassy's territory around 9am on Sunday morning and caused no damage or injuries, the Russian foreign ministry said in a statement on Monday.
"A large labelled fragment of the shell was later extracted from the earth by Syrian mine clearance specialists," the ministry's statement added. "Efforts are now being taken to find out how this shell came across to militants."
The ministry blamed the "criminal attack" on "anti-government gunmen" who it claims fired the shell from Jobar, a district of the Syrian capital.
The ministry called for "concrete action," adding: "We expect a clear position over this terrorist attack from all members of the international community, including regional parties."
The attack comes at a time when Russia has been criticised for increased military involvement in Syria, where it is a staunch ally of embattled President Bashar al-Assad.
The Russian government has admitted to having military advisers on the ground in Syria and delivering surface-to-air missiles and combat aircrafts to Syrian government forces, as well as refortifying a government-controlled military base near the coastal city of Latakia.
Russian leader Vladimir Putin reiterated his country's support for Assad's forces on Sunday. Speaking on Russian television, Putin said the Syrian government "is trying to maintain its own statehood".
The Russian foreign ministry's statement claimed the groups behind the attack were acting at the behest of "foreign sponsors", although it did not clarify which countries. No groups have claimed credit for the shelling at this time.
Rebels have denounced the ongoing Russian military buildup in Syria. Speaking to Reuters, one group, Jaish al-Islam, claimed to have carried out a missile attack on a military base in Latakia being used by Russian soldiers.
Abu Ghaith al-Shami, spokesperson for the Free Syrian Army-aligned Alwiyat Seif al-Sham militia, told Reuters that rebels' regional backers may also increase their involvement.
"As for the states that support us, ... I think there will be a change in their attitude towards us, via support, or perhaps a political shift," the spokesperson said.