The U.N. says the jet's downing may be a war crime, but the crash site is inaccessible.
"President Obama has no situational awareness regarding the adversaries he faces."
The German leader said it's "perhaps premature to draw conclusions" on the crash in Urkaine.
Ukrainian President Petro Poroshenko has accepted an offer of American assistance.
The reports come less than a week after a Malaysian jet was shot down.
"Direct talks between the opposing sides must be established as soon as possible," he said.
The president says his national security team is "in close contact" with Ukraine.
Kremlin says Russian president informed Obama "of the report from air traffic controllers."