President Barack Obama on Wednesday offered US assistance to Turkey after 41 people were killed in a massacre on Istanbul airport, the latest in a string of attacks to rock the US ally.

Obama telephoned President Recep Tayyip Erdogan "to express his deep condolences on behalf of the American people," White House spokesman Josh Earnest told reporters traveling with the US leader to a summit in Ottawa.

"In the context of that call, he will offer any support that the Turks can benefit from as they conduct this investigation."