USA-backed rebels mortar shells the Russian Embassy in Syria

What action should Assad take to protect foreign embassies?  What action should Russia take to protect its embassy? Should they file a complaint against the USA and others who are responsible for supplying arms to rebels with which they attacked the embassy?  Where would they file such a complaint?  The UN is in the USA's back pocket .... so what happens now?  Does Russia have a legitimate reason to tear with full force into the "opposition to Assad"?   What now?

Jamal Halaby and Jim Heintz of AP at OttawaCitizen:
....A mortar round landed inside the Russian Embassy compound in Damascus on Sunday, state media said, as Moscow said it was willing to send military observers to secure Syrian efforts to surrender its chemical weapons to international control.

Russia is a leading backer of Syria's President Bashar Assad and rebels fighting to topple his regime have previously targeted the diplomatic mission in Damascus with rockets and mortars.

In Moscow, the Russian Foreign Ministry confirmed the attack, saying rebels fired mortars that landed on the grounds of their embassy. Three workers were injured in the attack, the ministry said, adding that the injuries were not life-threatening. It gave no further details................

From RTNews:
Mortar shell hits Russian embassy compound  in Damascus, injures 3.  A mortar shell has hit the Russian embassy compound in the Syrian capital Damascus, injuring three people. The Russian Foreign Ministry and Syria’s authorities are taking measures to ensure additional security.

“On the morning of September 22, as a result of mortar shelling in the Damascus Mazraa district, one of the shells exploded on the territory of the Russian Embassy in Syria,” said Russia’s Ministry of Foreign Affairs in a statement adding that the three injured employees do not have life-threatening conditions.
The upscale Mazraa district also houses several Syrian security institutions, a soccer stadium and nightclubs.
Earlier reports suggested that there were two shells and several people were wounded, but they were not confirmed.
"Russia's embassy is working normally," the embassy's guard on duty stressed earlier, as cited by Itar-Tass. 

The US Department of State has condemned the shelling of the Russian embassy, expressing its “concern for the welfare of those injured in the incident.”
“We condemn any attack against individuals or facilities protected by international law. The United States continues to emphasize that those responsible for atrocities on all sides must be held accountable,” said a press statement published on the State Department’s website.............