MOSCOW — Russia and Turkey vowed on Tuesday not to let the assassination of a top Russian diplomat by an off-duty Turkish police officer derail their work or their fight against terrorism. Along with Iran, they pledged to expand a fragile cease-fire deal involving Syria’s government and the opposition.

A team of 18 Russian investigators landed in Ankara to look into the killing of the envoy, Ambassador Andrey G. Karlov, whose body was to be returned to Russia on the same plane in which the investigators arrived.

In Moscow, the Russian foreign minister, Sergey V. Lavrov, and his Turkish counterpart, Mevlut Cavusoglu, placed flowers next to a portrait of Mr. Karlov, who was fatally shot on Monday at a photography exhibition at an art center in Ankara, the capital.

“Turkish people are mourning this loss as much as Russia and the people of Russia,” Mr. Cavusoglu told Mr. Lavrov at the ceremony. Mr. Lavrov said that Russia was “grateful to our Turkish colleagues” for their condolences and for their rapid response to the killing.