'What exactly are they trying to hide?' Obama says of Russiaand Ukrainian rebels

Hrabove, Ukraine (CNN) -- Ukrainian President Petro Poroshenko and U.S. President Barack Obama lashed out Monday at Russia over conditions at the crash site of Malaysia Airlines Flight 17, saying Russian-backed rebels still are impeding efforts to find out exactly what happened.

Poroshenko, speaking to CNN's Christiane Amanpour, pleaded for international solidarity against the pro-Russian rebels believed by many international officials to be responsible for firing the missile that downed the plane Thursday, killing all 298 aboard.

"I don't see any differences" between 9/11, the Lockerbie bombing and the attack on Flight 17, Poroshenko said, referring to the 2001 terror attacks on the United States and the bombing of a Pan Am flight over Scotland in 1988.