According to various news reports, a Malaysia Airlines Boeing 777 flying from Amsterdam to Kaula Lumpur with 295 passengers has gone down in Ukraine near Ukraine's border with Russia. Malaysian Airlines is confirming that lost contact with the flight:

Malaysia Airlines has lost contact of MH17 from Amsterdam. The last known position was over Ukrainian airspace. More details to follow.
— @MAS
Reports from Ukraine's Interfax news agency say Ukrainian officials say plane was was shot down with a surface to air missile.
AP adds:
KIEV, Ukraine (AP) – An adviser to Ukraine’s Interior Minister says a Malaysian passenger plane carrying 295 people has been shot down over a town in the east of the country.
Anton Gerashenko says on his Facebook page the plane was flying at an altitude of 10,000 meters (33,000 feet) when it was hit Thursday by a missile fired from a Buk launcher.
A similar launcher was seen by Associated Press journalists near the eastern Ukrainian town of Snizhne earlier Thursday
According to MSNBC, Russia is denying involvement.
9:22 AM PT: NBC's Jim Miklaszewski says U.S. officials are pursuing the theory that the plane was shot down as "a likely option" but haven't reached any conclusion. If the plane was shot down, he says, U.S. officials believe that in that region, only the Ukrainian and Russian militaries have the missiles required to shoot down a plane flying at that altitude.
9:31 AM PT: Officially, Ukraine's president isn't assuming that the plane was shot down:
Ukraine’s president, Petro O. Poroshenko, said in a statement that he was calling for an immediate investigation of the crashed flight, a Boeing 777. He did not rule out that it might have been shot down.
9:45 AM PT:

Per Interfax, Putin and Obama have just spoken on the phone about Ukraine.
— @EllenBarryNYT
9:49 AM PT: Regarding Obama and Putin talking on the phone, MSNBC's Andrea Mitchell says the call probably was unrelated to the plane incident and may have taken place before the president knew about flight's disappearance.
10:04 AM PT: And another update: WH Press Secretary Josh Earnest says Putin brought up reports of the downed flight at the end of their call.