Insurgents fired two shells at the Russian embassy in the Syrian capital on Tuesday as hundreds of pro-government supporters gathered outside the compound to thank Moscow for its intervention in Syria.

An Associated Press reporter was outside the embassy when the first shell slammed into the compound in central Damascus and smoke billowed from inside. As people started running away, another shell hit the area.

It was not immediately clear if there were casualties.

Opposition fighters in the suburbs of the capital have targeted the embassy in the past but it was not clear if Tuesday's attack targeted the rally.

Before the shelling, the demonstrators had gathered outside the embassy carrying posters of the Russian and Syrian presidents, Vladimir Putin and Bashar Assad, and waved the two countries' flags.

Some held placards that read: "Thanks Russia" and "Syria and Russia are together to fight terrorism."

"President Putin's stances were absolutely positive for Syria," said 39-year-old civil servant Nizar Maqssoud.