BEIRUT, Lebanon — Two mortar shells struck the grounds of the Russian Embassy in Damascus, the Syrian capital, on Tuesday, an attack that Russia’s foreign minister called “an act of terrorism.”

The explosions, reported by Russian state news media and journalists on the scene, came as crowds of Syrians gathered at the embassy in a show of support for Russia’s new campaign of airstrikes on behalf of the Syrian government. There were no immediate reports of casualties.

“This is an obvious act of terrorism, probably aimed at terrifying supporters of the fight against terrorism,” Russia’s foreign minister, Sergey V. Lavrov, said as he prepared to meet the United Nations special envoy for Syria, Staffan de Mistura, in Moscow. He added that Russia was working with that Syrian authorities to find and punish the perpetrators.