"Apparently, it's a passenger airliner indeed, truly shot down by the Ukrainian Air Force,'' separatist leader Alexander Borodai told Russia's state-run Rossiya 24 TV broadcaster.
(For updates from verified social media accounts about the situation, scroll to this article's bottom.)
Another separatist, Andrei Purgin, told The Associated Press that he was certain that Ukrainian troops had shot the plane down but gave no explanation or proof.
Read MoreUkraine air space still open; but airlines avoid
Purgin said he did not know whether rebel forces owned Buk missile launchers, but said even if they did, there had no fighters capable of operating it.
There were unconfirmed reports that the crashed plane's flight recorder (the so-called black box) had been located.
U.S. President Barack Obama emphasized in a phone call with Poroshenko that all evidence from the crash site must remain in place until international investigators are able to examine all aspects of the tragedy. Meanwhile, UN Chief Ban Ki-Moon urged a "full and transparent international investigation" into the crash.
Pro-Russian separatists said they would have up to a three-day ceasefire in Eastern Ukraine to allow for recovery work at the site of the downed airliner, according to a RIA news agency.
Earlier, Obama said the U.S. is trying to determine if U.S. citizens were on board the crashed plane following reports that U.S. citizens were on board the plane.
Malaysia Airlines senior vice president Huib Gorter said there were at least 154 Dutch, 27 Australians, 23 Malaysians, 11 Indonesians, six Britons, four Germans, four Belgians, three Phillippinos and one Canadian aboard the Boeing 777 jet.
There were a further 47 passengers whose nationality was not yet known. All the 15 flight crew were Malaysian.
Calling the downed plane a "terrible tragedy," Obama said the U.S. will offer any help it can to determine what happened and why.
In a phone conversion, Putin told Obama that a Malaysian jetliner crashed on Ukrainian territory, the Kremlin said. The two leaders held a already planned call on the situation in Ukraine following expanded sanctions on Russia during which information became available from air traffic controllers about the plane.
Read MoreObama warns Putin of more sanctions over Ukraine
Ukraine's prime minister identified the shot-down plane as a missing Malaysian Airlines flight and has ordered an investigation. A YouTube video purported to show footage from moments after the plane went down.
Malaysian Prime Minister Najib Razak said he was shocked by the reports and was also planning an investigation.
On his Facebook page, Ukraine's minister of internal affairs, Anton Gerashchenko, claimed that terrorists struck down the plane under Putin's orders.