MOSCOW, December 20 /TASS/. US Ambassador to Russia John Tefft has offered condolences to Russia over the murder of Ambassador Andrei Karlov in Ankara on Monday.

"He was a professional who died doing his hard but necessary work of a diplomat," Tefft wrote on the US embassy page on Twitter. "Rest in peace," Tefft said.

The Russian embassy in Washington will open a Condolence Book at 14:30 (2:30 p.m.) local time on Tuesday.

The Russian diplomats said that the condolences had already "started to arrive." The White House, the State Department and the Pentagon have already made corresponding statements. Ordinary Americans have brought flowers to the Russian embassy’s fence.

Russia’s ambassador to Turkey Andrei Karlov was gunned down in Ankara late on Monday as he was opening a photo exhibition titled "Russia: From Kaliningrad to Kamchatka, through the eyes of a traveler."

According to Ankara, Turkish security services liquidated the assassin who himself turned out to be a policeman. Three people were injured in the incident, which the Russian Foreign Ministry described as a terror attack. The Russian Investigative Committee, which initiated criminal proceedings, classified Karlov’s murder as an act of international terrorism.