"We extend our deepest condolences to the loved ones of those on board Malaysia Airlines Flight MH370," State Department Spokesperson Jen Psaki said.
"At this time, we can confirm that three US citizens were on board. Officials from the US Embassies in Kuala Lumpur and Beijing are in contact with the individuals' families. Out of respect for them, we are not providing additional information at this time," she said.
"The Embassies are working to assess whether additional US citizens may have been on board the flight," Psaki said in a statement.
Malaysia Airlines said the last point of contact with the plane was approximately 120 nautical miles east of Kota Baru in South China Sea.