Aleksander Borodai, a leader of pro-Russian rebels, handed over the two black box recorders from Flight MH17 shot down over Ukraine to Malaysian officials Tuesday morning. (Reuters)

Aleksander Borodai, a leader of pro-Russian rebels, handed over the two black box recorders from Flight MH17 shot down over Ukraine to Malaysian officials Tuesday morning. (Reuters)

After days of resistance, pro-Russian rebels on Monday yielded some ground in the crisis surrounding downed Malaysia Airlines Flight 17 — handing over passengers’ bodies, relinquishing the plane’s black boxes and pledging broader access for investigators to the crash site.

The developments offered some hope that an international investigation might clarify how the civilian jet carrying 298 passengers and crew members was shot down Thursday over territory held by the separatists in eastern Ukraine. Experts warned, however, that the site had been compromised.

The breakthroughs came after days of international outrage over scenes of bodies decaying in meadows under a hot sun. The U.N. Security Council and world leaders had demanded that the rebels allow professional investigators unfettered access to the site.

Still, underlining the intensity of the broader conflict, combat continued in eastern Ukraine. A rebel leader said he was skeptical about discussions to reach a temporary truce in the fight with the pro-Western government of Ukraine, which would have made it easier for experts to study the crash site.

For its part, the Ukrainian military on Monday pressed an assault near Donetsk’s city center, 40 miles from the area of the crash. Artillery strikes hit targets near the Donetsk railway station and airport, including residential buildings, witnesses said. The government denied aiming at civilian facilities.

1 of 63 Full Screen Autoplay Close July 30, 2014 July 28, 2014 July 25, 2014 July 24, 2014 July 23, 2014 Wednesday July 22, 2014 Tuesday July 21, 2014 Monday July 20, 2014 Sunday Skip Ad × Picking up the pieces from Malaysia Airlines Flight 17 View Photos As hundreds of volunteers were combing the crash site for bodies and debris, friends and family members were trying to cope with their loss. Caption As volunteers were combing the crash site for bodies and debris, friends and family members were trying to cope with their loss. July 31, 2014 Commander Brian McDonald, left,of the Australian Federal Police, Alexander Hug, center, deputy head for the Organisation for Security and Cooperation in Europe's (OSCE) monitoring mission in Ukraine and, right, Dutch police officer Kuijs return from the MH17 crash site. Sergei Karpukhin/Reuters Buy Photo Wait 1 second to continue.

The Malaysia Airlines jet, the recovery of passengers’ bodies and the ensuing investigation have all fallen victim to the conflict that has raged since spring between the Ukrainian government and separatist groups that are armed, and in some cases led, by Russians. The Malaysian airliner was struck by a missile that Ukraine, the United States and many other governments believe was fired by separatists, though the rebels and Moscow have denied it and blamed Kiev instead. The rebels had allowed onlookers to roam around the crash site and had limited access to investigators while they dickered with international and Ukrainian authorities seeking to retrieve the bodies and assess evidence.

Early Tuesday, the rebels handed over what they said were the plane’s black boxes. The transfer took place in a ceremony with a Malaysian delegation in the rebel-controlled regional administration building in Donetsk.

The train carrying the bodies of Flight 17’s passengers is expected to arrive in Kharkiv around midday Tuesday, some 18 hours after it left the town of Torez near the crash site.

A team of forensics experts from the Netherlands, France, Malaysia and Australia -- all countries whose citizens died in the crash -- are awaiting its arrival at a closed military base in the city of Kharkiv, said Esther Naber, a spokeswoman for the team.

The forensics experts will take the 282 bodies and prepare them more properly for transport back to the Netherlands, she said. That involves placing the victims’ remains in better body bags and laying them in sealed coffins, she said.

“That’s going to take some time,” she said, “because of the sad fact we have so many victims.”

While it is possible the task could be completed by Tuesday evening, it is more likely that the bodies will not be flown back to the Netherlands until Wednesday morning, she said. All the bodies will be taken to the Netherlands for identification.

View Graphic Satellite image of the debris field near Hrabove

Late Monday, Malaysian Prime Minister Najib Razak said he had reached an agreement with Alexander Borodai, a rebel leader, for a Malaysian team to take custody of the two black-box data recorders from rebel fighters who had retrieved them and previously refused to turn them over. He also said international investigators would be guaranteed safe access to the crash site.

Meanwhile, three Dutch investigators, whose access to the crash site was previously blocked by the Russian-backed separatists, began gathering evidence Monday.

In an interview with CNN, Borodai insisted the separatists were eager for the bodies to be collected quickly. He said the rebels had been hampered by statements from the Organization for Security and Cooperation in Europe (OSCE) that the separatists were responsible for any bodies that were moved.

“It got to the point where it resembled, if not a horror movie, then black humor,” he said. “When an old woman comes to our rebel groups and says: ‘Look, there is a body of a headless man [that] fell through the roof straight onto my bed. Please take this man away.’ But the rebels say no, because they are following instructions.”

Underscoring the antagonism that contributed to the delays, Borodai told reporters in Donetsk that the bodies and the objects that the rebels believed to be the black boxes would be handed over to “foreign experts, but not to the Ukrainian side.”

He said that he was doubtful that talks with the OSCE on a temporary truce with the Ukrainian military would yield fruit.

“I am not very optimistic about this meeting,” he said. “The previous ones had no results.”

Even as final negotiations progressed over the train carrying the bodies, the Ukrainian military attacked the center of Donetsk. Its central train station was partially evacuated for several hours, although trains continued to run and the facility was not damaged, a representative of the station said.

Ukrainian military authorities made no apologies for carrying out an assault just miles from where the team of international observers was gathering to inspect the scene of the plane crash.

“This is a planned offensive,” said a military spokesman, Vladislav Seleznev. The military was trying to push rebels away from the airport, he said. “Aviation and artillery are not aiming at civilian residences. Their only aim is to block the terrorists and fighters.”

Ukrainian President Petro Poro­shenko announced that the investigation into the crash would be based in the Netherlands.

In remarks carried by the Ukrainian Interfax news agency, he described the rebels as “barbarians” who murdered 298 innocent people and then looted children’s toys from the luggage that dropped from the sky. He said the rebels had committed three crimes — shooting down the plane, treating the bodies with negligence and disrespect, and tampering with evidence.

In Kharkiv, where the train bearing the bodies was headed, the focus was on getting the victims’ remains home.

“I’m here because I hope I can help,” said Marina Kravchenko, an English-language teacher who volunteered to work at a government call center fielding inquiries from relatives of the passengers. “I don’t know what else to do.”

Morello reported from Kharkiv. Annie Gowen in Kuala Lumpur, Natasha Abbakumova and Karoun Demirjian in Moscow, and Karen DeYoung in Washington contributed to this report.