Hrabove, Ukraine (CNN) -- Pro-Russian rebels in eastern Ukraine handed over the black box data recorders from downed Malaysia Airlines Flight 17, days after the passenger jet crashed.
The self-declared rebel leader in Donetsk said at a ceremony early Tuesday that they were giving the black boxes to the Malaysian government with the understanding that the information would be shared with aviation experts around the world.
And he said that separatists had done the best they could to retrieve bodies and handle the wreckage.
The long-awaited handover came hours after Ukrainian President Petro Poroshenko and U.S. President Barack Obama lashed out Monday at Russia over conditbions 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.
Obama called on Russia to rein in the rebel fighters, who he said had treated remains poorly and removed evidence from the site.
"What exactly are they trying to hide?" he said.
Obama said it was time for Russia to exert what he called its "enormous influence" over the rebel fighters -- who U.S. and other officials have say are armed, trained and backed by Russia -- to persuade them to better cooperate with the international investigation.
"It's the least they can do," he said.
The U.N. Security Council adopted a resolution Monday demanding full access to the crash site and condemning the downing of the plane. The resolution won unanimous approval from the 15-member council, which includes Russia. It did not specify who was responsible for the crash.
Despite the stern tone of the Ukrainian and U.S. leaders, the spokesman for a team of European monitors at the site said conditions have improved since a chilly reception immediately after Flight 17 fell from the sky.
"Today we have three Dutch forensics experts with us, and they're getting pretty much unfettered access," Michael Bociurkiw, the spokesman for monitors from the Organization for Security and Co-operation in Europe, told CNN's Chris Cuomo.
Dutch forensic experts and a handful of Ukrainian aviation experts worked the scene Monday, Bociurkiw said separately in a briefing for reporters hosted by the Ukrainian Crisis Media Center.
Handling the remains
The remains of 16 people were still missing Monday, four days after Flight 17 plunged to the ground, Poroshenko told Amanpour.
Earlier, the Ukrainian government issued a statement saying that 282 bodies and 87 "body fragments" had been recovered from the sprawling crash site.
A train carrying the remains of 251 passengers was expected to arrive in the eastern city of Kharkiv by midnight, Ukrainian officials said Monday. It will first have to pass through Donetsk, the scene of fighting earlier in the day between rebels and government forces.
Obama and Poroshenko decried how the bodies had been treated, echoing complaints that the remains had been left exposed to the elements for days and that rebels had stripped personal belongings from some of the bodies and their effects.
Poroshenko said the rebels' conduct was "barbaric." Obama called the handling of remains an "insult" that has "no place in the community of nations."
Dutch Foreign Minister Frans Timmermans said bringing the victims' remains home is his country's top priority.
"To my dying day, I will not understand that it took so much time for the rescue workers to be allowed to do their difficult jobs," he told the U.N. Security Council on Monday, "and that human remains should be used in a political game."
Dutch forensics experts who inspected the train Monday were "more or less" satisfied with how the bodies were being stored," Bociurkiw said.
Ukrainian government officials have said the bodies will eventually be taken to Amsterdam. Most of those who died in the crash were Dutch.
Dutch Prime Minister Mark Rutte met with relatives and friends of victims Monday, calling the session filled with sadness and "very touching."
"All of the Netherlands is feeling their fury. All of the Netherlands is sharing their deep sadness, and all of the Netherlands is just gathering around all the next of kin," he said.
Bociurkiw had no information about the status of a team of international crash experts staging in Kharkiv to inspect the debris. Earlier, the Ukrainian government issued a news release saying the experts had reviewed photos of the crash scene.
Another team from the Netherlands remains in Kiev, according to the Dutch Foreign Ministry, and some Malaysian investigators flew to the Ukrainian capital of Kiev on Saturday. But Malaysia's official news agency said they were still negotiating with rebels over access for their team.
The United States has sent two FBI agents, according to a senior U.S. law enforcement source. An investigator from the National Transportation Safety Board was also in Kiev. Law enforcement officials from the Netherlands and Australia were also expected.
'An outrage made in Moscow'
U.S. and other officials have said it appears the plane was shot down by a sophisticated surface-to-air missile located within rebel-held territory. Evidence supporting that conclusion includes telephone intercepts purporting to be pro-Russian rebels discussing the shootdown and video of a Buk missile launcher traveling into Russia with at least one missile missing.
While they have stopped short of putting the responsibility squarely on Russia, Obama, British Prime Minister David Cameron and others have said the pro-Russian rebels could not have shot such a high-flying jet down without weapons and training from Russia.
In an op-ed in The Sunday Times, British Prime Minister David Cameron called the plane crash and its aftermath "an outrage made in Moscow."
But officials said Monday that U.S. intelligence analysts are examining phone intercepts, social media posts and information gathered on the ground to see if Russian officials played a direct role in the shootdown, according to two U.S. officials directly familiar with the latest assessment. The officials declined to be identified because of the sensitivity of the situation.
"We are trying to determine if they manned it, advised, or pulled the trigger," one of the officials told CNN.
Russians blame Ukraine
Moscow has strongly denied the claim, and on Monday, a Russian general suggested that it may instead have been a Ukrainian jet fighter that shot the plane down.
Russian monitoring showed a Ukrainian Su-25 fighter jet flying along the same route and within 3 kilometers to 5 kilometers (1.9 miles to 3.1 miles) of Flight 17, Lt. Gen. Andrei Kartapolov of the Russian Army General Staff said at a news conference, Russian state media reported.
"We would like to know why the Ukrainian plane was flying along a civilian route on the same flight path as the Malaysian Boeing," Kartapolov said, according to the reports.
In his interview with Amanpour, Poroshenko rejected the Russian suggestion, saying all Ukrainian aircraft were on the ground at the time.
Pro-Russian rebels have also denied responsibility for the shootdown.
In an interview with Cuomo broadcast Monday on CNN's "New Day," the self-declared rebel Prime Minister in Donetsk, Alexander Borodai, said he believed Ukrainian forces either shot the plane down with a surface-to-air missile or, as the Russian general suggested, one of its own fighter jets.
"We didn't have motives and desire to do that, and it is obvious that Ukrainians have them," he said. "I can't say about desire, but motive is obvious that the crash of this plane was beneficial to them."