Syrian rebel fighters 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.

The incident came nearly two weeks after Russia, a major ally of Syria's President Bashar Al-Assad, launched airstrikes in the country, aimed, Moscow says, at members of the Islamic State in Iraq and the Levant (ISIL) but which have targeted a wide array of anti-Assad groups.

One witness said both shells appeared to land in a park close to the embassy compound. A second witness said one of them landed inside the compound but did not hit the building itself.

Russia's Interfax news agency said two rockets landed in the embassy grounds. It quoted a diplomat at the embassy as saying none of the embassy employees was wounded.

It was not immediately clear if there were any casualties.

"This is an obvious act of terrorism, probably aimed at intimidating supporters of the fight against terrorism," Russian Foreign Minister Sergei Lavrov was quoted as saying ahead of talks with the United Nations special envoy for Syria, Staffan de Mistura, on Tuesday.

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

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

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

Russia began launching airstrikes in Syria on Sept. 30.

While Moscow insists it is mainly targeting ISIL and other "terrorists," the multi-pronged ground-and-air offensive is being waged in areas controlled by U.S.-backed rebels as well as other groups including Nusra Front, Syria's Al-Qaeda affiliate.

Earlier Tuesday, the Nusra Front released an audio message purportedly from its leader describing Russian military intervention as a new "Crusader campaign" aiming to save Assad's rule.

The Nusra Front leader known as Abu Muhammed al-Golani called on Syrian rebel groups to unite and intensify shelling of villages inhabited by members of Assad's minority Alawite sect.

Golani also called on Muslims in the former Soviet Union to attack Russian civilians if Russians target civilians in Syria.

"The Russian intervention came to declare a new eastern Crusader war after the western Crusader war failed in Syria," Golani said, in an apparent reference to airstrikes by the U.S.-led coalition that began last year. Golani added that the Russians are not targeting ISIL fighters as they claim but are striking at rebel groups who are fighting the government.

Golani promised to pay $3.42 million to whomever kills Assad and $2.28 million to whomever kills Hezbollah leader Sheikh Hassan Nasrallah, whose men are fighting along with Assad's forces.

Al Jazeera and wire services