Terming the loss of the plane "a terrible tragedy", President Barack Obama, who was on a previously scheduled visit to Delaware, said the US would help in every way possible to try to determine exactly what happened. A first priority was to determine how many American passengers might have been on board. Reports said that there were 23 Americans on the passenger manifest.
The crash came just one day after Washington imposed a new layer of economic sanctions on Russia for allegedly continuing to stoke the rebellion by pro-Russian activists in eastern Ukraine. Among US complaints is that Moscow has allowed an unimpeded flow of weaponry across the border, possibly including surface-to-air missile launchers.
Earlier, Moscow had made public its dismay at the new US sanctions, with sharp comments, including from President Vladimir Putin.
"As far as sanctions are concerned, they usually have a boomerang effect, and will lead US-Russia relations down a dead end," Putin said. "It's a great pity that our partners are following this path. But our doors are open for a negotiating process, for a way out of this situation."
Earlier Obama spoke with Putin by telephone. The call, which had been requested by Moscow, was ostensibly about the sanctions. However, officials in Moscow said Putin informed Obama about the loss of the Malaysian aircraft.
The US had already made clear that still tougher sanctions were on the table in case Russia fails to defuse the situation in Ukraine. Should any evidence emerge that the jet was hit by weaponry fired by pro-Russian forces in that country and supplied by Moscow the chances of additional measures being imposed will surely grow dramatically.
Even more serious measures would be considered, however, were it ever to be determined that it was shot down by Russian forces, whether intentionally or accidentally.
Military experts believe the Malaysian jet could have been downed by a Russian-built mobile missile launcher capable of hitting targets up to 80,000ft (24,384m) in the sky.
The Buk missile battery is a Soviet-era weapon equipped with a 70kg warhead, designed for taking out cruise missiles, aircraft and drone planes. Usually mounted on a tracked vehicle, it is in use in both Russia and Ukraine.
Yesterday, one missile battery was reported in territory held by militiamen in Ukraine's rebel-held east. While the separatists are known to have other anti-aircraft systems in their arsenal - and are believed to have downed two Ukrainian aircraft earlier this week - only the Buk has the range to hit an airliner above 30,000ft.
Leaders react
International leaders and officials expressed grief and shock at yesterday's crash in eastern Ukraine. Several pledged to contribute to investigation efforts.
Mark Rutte, Dutch Prime Minister
"Possibly one of the worst air disasters in Dutch history ... I am shocked. I am broken up. All of us in the Netherlands are in deep mourning."
Najib Razak, Malaysian PM
"We must - and we will - find out precisely what happened to this flight. No stone can be left unturned. If it transpires that the plane was indeed shot down, we insist that the perpetrators must swiftly be brought to justice. This is a tragic day, in what has already been a tragic year, for Malaysia ... The flight's passengers and crew came from many different countries. But today, regardless of nationality, we are all united in grief."
Petro Poroshenko, Ukraine
President Poroshenko called it an "act of terrorism" and demanded an international investigation.
Vladimir Putin, Russian President
"This tragedy would not have happened if there were peace on this land, if the military actions had not been renewed in southeast Ukraine. And, certainly, the state over whose territory this occurred bears responsibility for this awful tragedy."
Tony Abbott, Australian PM
"We owe it as well to the families of the dead to find out exactly what has happened and exactly who is responsible ... As things stand, this looks less like an accident than a crime. And if so, the perpetrators must be brought to justice."
John Kerry, United States Secretary of State
"We are horrified by the crash of Malaysia Airlines flight MH17. There are no words adequate to express our condolences to the families of the nearly 300 victims. We offer our sympathies and support to the Governments of Malaysia and the Netherlands at this difficult time, as well as to all those whose citizens may have been on board. We are reviewing whether any American citizens were aboard the flight. The United States Government remains prepared to assist with a credible, international investigation any way we can, and we will continue to be in touch with all relevant partners as we seek the facts of what happened today."
Ban Ki-Moon, United Nations Secretary-General
"There is clearly a need for a full and transparent international investigation."
Chris Beyrer, president-elect of the International Aids Society
Commenting on reports that world-renowned Dutch HIV researcher Joep Lange was on Flight 17: "Then the HIV/Aids movement has truly lost a giant."Anwar Ibrahim,Malaysian opposition leader "This is yet another national tragedy and our moment of deep grief and sorrow."
- Independent, Telegraph Group Ltd, AP
