KIEV, Ukraine — The quest for clues in the downing of a Malaysia Airlines jet faced major obstacles Friday as the rebel militias that Ukraine blames for the disaster limited access to the mammoth crash site strewn with airplane parts and bodies.

Ukrainian officials were trying to negotiate safe passage for teams of investigators and international observers deep in territory held by the pro-Russian separatists in the east of the country.

The fate of the airplane’s black-box data recorders, which could give crucial information about the plane’s final moments, was unclear, with neither side acknowledging possession. And rebels argued among themselves whether to agree to a brief cease-fire to allow the bodies of victims to be taken to morgues elsewhere in the country.

Rebel leaders said they would leave victims and airplane parts in place to facilitate the investigation and vowed to allow investigators to visit the site because, they said, they had nothing to hide in the catastrophe in which 298 people died.

Ukrainian officials moved swiftly Friday to link the plane disaster to the rebels, saying that the Boeing 777-200 had been downed by separatists using a surface-to-air missile, possibly with direct Russian aid. President Obama said Friday that U.S. intelligence indicates that a Russian-made missile downed the plane from rebel territory, but he stopped short of saying who pulled the trigger.

“Nearly 300 innocent lives were taken – men, women, children, infants who had nothing to do with the crisis in Ukraine,” Obama said at the White House. “Their deaths are an outrage of unspeakable proportions.”

Ukrainian Prime Minister Arseniy Yatsenyuk echoed that sentiment in a highly emotional address posted on his website. “This is a crime against humanity. All red lines have already been crossed,” he said. “After these terrorists shot down a Malaysian Airlines aircraft, this is a war against the world.”

Both rebel leaders and Russian officials denied any connection to the crash, and Russian Foreign Minister Sergei Lavrov said Friday that Ukrainian authorities bore broad responsibility for creating conditions in which citizens were moved to rebellion. He did not suggest that the Ukrainian military had shot down the plane.

“What happened with the aircraft should make us stop, look back, and reflect” on the situation that began when Russia-friendly President Viktor Yanukovych was ousted in February, Lavrov said.

The extensive debris zone includes not only the wheat field where most of the plane was found but also nearby villages. Witnesses described looking across beautiful fields of sunflowers only to be jolted by the discovery of body parts on the ground.

The recovery efforts were moving slowly amid the civil conflict, and each step forward brought new tensions among the parties, many of whom are heavily armed.

The Organization for Security and Cooperation in Europe, the Vienna-based organization coordinating a dialogue in the conflict, sent a group of 30 observers to the crash scene Friday, said Shiv Sharma, a spokesman.

The monitors were given access to the site for 75 minutes, and while they were at the heavily guarded site, rebels fired their weapons into the air, Sharma said.

“The shots were not targeting the monitors as such; they were just into the air. It was not a particularly tense situation in which monitors were concerned about their security,” he said.

But the U.S. State Department said in a Twitter message Friday that monitors “were only granted limited access” to the crash site. “They must have complete unfettered access,” the message said.

The observation team is negotiating on a day-to-day basis for time at the site. Its job is to secure the crash scene until independent investigators arrive to help with the transfer of bodies. As of Friday evening, 181 bodies had been found, according to a Ukrainian foreign ministry official, Andrii Sybiga.

The international team, which includes FBI personnel and a National Transportation Safety Board investigator from the United States, was expected to re-enter the area later Friday or Saturday morning.

The rebel leaders, who often argue among themselves, were divided over whether to agree to a cease-fire to allow for a fuller international presence at the scene.

“We’re highly interested in an unbiased and full investigation and are prepared to give unrestricted access to the spot of the crash for experts of the CIS Interstate Aviation Committee,” said separatist leader Alexander Borodai, according to the Russian news agency ITAR-TASS. Borodai was referring to the Commonwealth of Independent States, a group of former Soviet republics. Access for the committee would give Russian investigators on the scene a major role.

By day’s end, Borodai said no truce talks were being held, and another rebel leader, Denis Pushilin, announced from Moscow that he was resigning from leadership, the Interfax news agency reported.

Rebels said they would allow investigators to take victims’ bodies to morgues in government-held territory, saying that they did not have enough refrigeration capacity to hold all the bodies.

Underlining how difficult any investigation will be, violence has continued even after the plane crash, particularly in Ukraine’s far-eastern Luhansk region.