President Barack Obama says a full and unfettered investigation must be allowed to happen in eastern Ukraine after last week's crash of a Malaysian airliner.
In a statement at the White House, Obama accused Russian-backed separatists of impeding the investigation.
He asked, "What exactly are they trying to hide?"
Obama said Russian President Vladimir Putin has direct responsibility over the rebels and should compel them to cooperate with international investigators.
Russia is facing increasing international pressure over its actions in eastern Ukraine after the airline crash.
Nearly 300 people were killed in the crash, which is being blamed on a ground-to-air missile launch.
U.S. officials suspect the attack was launched by pro-Russian rebels.
The U.S. and allies have imposed economic sanctions on Moscow.
President Obama said the costs for Russia's behavior will only increase if Moscow does not change course in Ukraine.
Obama also insists that Israel has a right to defend itself against rocket attacks from Hamas.
The President expressed serious concerns about the loss of civilian life.
Secretary of State John Kerry has been dispatched to the Middle East to continue work on a ceasefire agreement between Hamas and Israel.
Hundreds of Palestinians have died as a result of shelling and an intensifying Israeli ground invasion in Gaza. Several Israeli soldiers have also been killed.