The below is attributable to Spokesperson John Kirby:
Secretary Kerry called Russian Foreign Minister Lavrov this morning to express his deep condolences on the assassination of Russian Ambassador Andrey Karlov yesterday in Ankara. He offered U.S. assistance to the investigation in whatever capacity would be required.
The Secretary and Foreign Minister also discussed the specific situation in Syria and the ongoing efforts to reach an immediate ceasefire, secure the prompt delivery of humanitarian assistance, and resume political talks to end the civil war.
Source: U.S. State Department.