International investigators began gaining access Monday to the site in eastern Ukraine where Malaysia Airlines Flight 17 crashed last week, as pro-Russia separatists holding the area turned over the plane’s flight recorders to Malaysian officials.

The signs of compromise came as the bodies of nearly all 298 killed in the disaster began making their way by train out of the war zone to the government-controlled city of Kharkiv.

The separatists, who have been fighting the Ukrainian government in recent months with the backing of Russian President Vladimir Putin’s government, are widely suspected of shooting down the plane. U.S. intelligence has determined that the missile that brought it down came from insurgent-held territory.

Monday’s events marked a potential turning point after three days in which separatists heavily restricted movement in the area, often with a show of weaponry. Whether the change for the pro-Russia group led by Oleksandr Boroday was prompted by internal dynamics, external pressure or new communication from Moscow was unclear.

Boroday and other separatist leaders handed the aircraft’s flight recorders — which, if undamaged and not tampered with, could give investigators information about the downing — to Malaysian officials late Monday in the insurgent-held city of Donetsk. There was a formal quality to the event, staged by the separatists and witnessed by invited journalists, with the Malaysian delegation calling Boroday “His Excellency.”

Boroday said he had been persuaded to hand over the recorders by Malaysian Prime Minister Najib Razak.

The announcement Monday evening that the train loaded with the bodies would leave the war zone shortly for Kharkiv came as a particular surprise: As recently as Monday afternoon, with separatist gunmen surrounding the rail cars as they sat at the station of the industrial town of Torez, many observers thought a lengthy negotiation awaited.

As experts and investigators from a number of countries, including the United States, made ready to move into the contested area, some world leaders worried whether the investigators would have freedom of movement and how much of the evidence at the crash sight might have been compromised by the separatists.

“It’s a little like leaving criminals in control of the crime scene,” said Australian Prime Minister Tony Abbott.

The United Nations Security Council adopted a resolution Monday calling for investigators to have unfettered access to the site and demanding a cease-fire in eastern Ukraine. The U.S. ambassador to the world body, Samantha Power, said a resolution would not have been necessary if Russia had used its influence over the separatists to permit access to the scene.

Russia joined other Security Council members in the 15-0 vote, and Russian Ambassador Vitaly Churkin rejected the U.S. allegations of Russian involvement in blocking access to the site. “There’s no need to turn the discussion of the tragedy into a farce,” he said of Power’s comments.

The administration of Ukrainian President Petro Poroshenko ordered a military safe zone of 25 miles around the crash site, and Boroday’s group was urged to follow suit. Fighting between government forces and the separatists continued Monday, particularly around Donetsk, where unconfirmed reports late Monday indicated that troops had retaken the airport.

Three Dutch forensic specialists on Monday became the first experts to arrive in the region. They joined monitors from the Organization for Security and Cooperation in Europe in inspecting the trains and visiting the spot where the plane’s cockpit struck the ground.

The Netherlands, which lost 193 people aboard the plane, will be taking over the investigation of the downed jet in conjunction with international air safety organizations, Ukrainian officials said.

In Washington, President Obama had strong words for Moscow and the group controlling the area, saying that if Russia continued to back the separatists, it would “only further isolate itself from the international community, and the costs for Russia’s behavior will only continue to increase.”

“We have to make sure that the truth is out and that accountability exists,” Obama said. “What exactly are they trying to hide?”

Putin offered his own remarks, in which he made no mention of the separatists and took no responsibility for the crash, saying that those who blamed him were using the disaster for political ends.

“No one should and no one has the right to use this tragedy to pursue their own political goals,” he said. “Rather than dividing us, tragedies of this sort should bring people together.”

Obama administration officials believe Putin’s willingness to help ensure the crash site is secure and open to investigators could indicate a shifting stance on Ukraine. After months of watching Russia profess support for peace talks while sending arms across the border into eastern Ukraine, U.S. officials expressed cautious optimism that the downing of the plane might mark a grim turning point.

A senior administration official said the U.S. intelligence community was increasingly confident that the separatists were responsible for the crash. Officials say they have little doubt that the separatists used an antiaircraft missile system, probably supplied by Moscow, and that they were trained and possibly led by Russian nationals.

Still, the administration official, who requested anonymity in discussing the sensitive issue, said the U.S. had not established that any specific Russian officials were at the missile launch site at the time of the incident.

Most of the crash victims were from Europe, and the continent’s foreign ministers will meet in Brussels on Tuesday to discuss toughening sanctions against Russia. British Prime Minister David Cameron and Dutch Prime Minister Mark Rutte are among leaders who want harsher measures more in line with the American approach.

“We should push our partners in the European Union to consider a new range of hard-hitting economic sanctions against Russia,” Cameron said Monday, adding that the European Union should move to the highest of three tiers of sanctions, in which entire sectors are targeted.

As global outrage grows over the separatists' handling of the crash site and Russia's‎ involvement with the paramilitary groups, Ukraine increasingly feels it has world opinion on its ‎side. On Monday, it sought to capitalize on that momentum, with Prime Minister Arseny Yatsenyuk calling a news conference in Kiev, the capital, to press the point.

“This is a global threat and Russia is on the dark side,” he told reporters. “This is our priority and [should be] the key priority of the entire world — to stop Russian aggression.”

He added: “President Putin has to realize enough is enough. This is not just a conflict between Ukraine and Russia. This is an international and global conflict.”‎

Times staff writers Henry Chu in London, Kathleen Hennessey in Washington, Tina Susman at the United Nations in New York and Sergei L. Loiko in Moscow contributed to this report.