As Kiev raised the stakes by saying it had evidence that a Russian fired the missile widely assumed to have killed all 298 aboard on Thursday, a separatist leader blamed Ukraine for the delay and called on Moscow to help in recovering bodies starting to rot after two days in baking summer heat on the steppe.
Russia urged both sides to open access to foreign experts.
After President Barack Obama called the loss of flight MH17 a "wake-up call" to Europe to join the United States in threatening Moscow with heavier economic sanctions if it does not help end the conflict, German Chancellor Angela Merkel spoke to Russian President Vladimir Putin, urging him to use his influence with the rebels to ensure an urgent ceasefire.
"Moscow may have a last chance now to show that it really is seriously interested in a solution," Merkel's foreign minister, Frank-Walter Steinmeier told Bild am Sonntag newspaper.
"Now is the moment for everyone to stop and think to themselves what might happen if we don't stop the escalation."
Germany, like other EU states heavily dependent on Russian energy and trade, has been less eager than Washington to damage its own economy by escalating a confrontation with Moscow that has revived memories of the Cold War. But with military action not seen as an option, Western powers have few other levers.
Fighting flared in eastern Ukraine overnight. The government said it was pressing its offensive near Donetsk and Luhansk.
Malaysia, whose national airline has been battered by its second major disaster this year, said it was "inhumane" to bar access to the site around the village of Hrabove, near Donetsk, but said Russia was doing its "level best" to help.
Observers from Europe's OSCE security agency visited part of the crash site near the village of Hrabove for a second day on Saturday and again found their access hampered by armed men from the forces of the self-declared People's Republic of Donetsk. An OSCE official said, however, they saw more than on Friday.
At one point, a Reuters correspondent heard a senior rebel tell the OSCE delegation they could not approach the wreckage and would simply be informed in due course of an investigation conducted by the separatists. However, fighters later let them visit an area where one of the Boeing 777's two engines lay.
"The terrorists, with the help of Russia, are trying to destroy evidence of international crimes," the Ukrainian government said in a statement. "The terrorists have taken 38 bodies to the morgue in Donetsk," it said, accusing people with "strong Russian accents" of threatening to conduct autopsies.
Ukraine's prime minister said armed men barred government experts from collecting evidence and threatened to detain them.
In the regional capital Donetsk, the prime minister of the separatist authorities told a news conference that Kiev was holding up the arrival of international experts whose mission to probe the cause - and potentially blame - for the disaster was authorised on Friday by the United Nations Security Council.
And contrary to earlier statements by the rebels, Alexander Borodai said they had not found the black box flight recorders. He said rebels were avoiding disturbing the area where the plane crashed, spreading corpses over many miles.
"There's a grandmother. A body landed right in her bed. She says 'please take this body away'. But we cannot tamper with the site," Borodai said. "Bodies of innocent people are lying out in the heat. We reserve the right, if the delay continues ... to begin the process of taking away the bodies. We ask the Russian Federation to help us with this problem and send their experts."
Midday temperatures are around 30 Celsius (85 Fahrenheit).
At Hrabove, one armed man from the separatist forces told Reuters that bodies had already been taken away in trucks. Amid reports of looting, fighters and local people say they have been doing their best to collect evidence and preserve human remains.
As the stench of death began to pervade the area, a Reuters correspondent watched rescue workers carry bodies across the fields and gather remains in black sacks. One local resident said Ukrainian fighter jets had flown over the area earlier.
Ukraine has accused the rebels of trying to spirit away the black boxes and the missile-launcher across the Russian border.
On Saturday, counter intelligence chief Vitaly Naida said he had "compelling evidence" that not only had the SA-11 Buk radar-guided missile system Kiev says was used to hit the airliner been brought over the border from Russia, but the three-man crew was also comprised of Russian citizens. He said the unit had returned to Russia and demanded Moscow let Kiev question them.
U.S. officials describe as convincing audio recordings that the Ukrainian government has released purporting to be of Russian officers and rebels discussing shooting down the plane.
Moscow has repeatedly denied Kiev's accusations that it is supply manpower and hardware across the frontier to the rebels.
The Kremlin said Putin insisted to Merkel that the inquiry should be objective. Locked in a propaganda war with Ukraine, Russia has suggested Ukrainian forces may have brought down the plane, an allegation made more concretely by rebel leaders.
The Russian Foreign Ministry said on Saturday Moscow would retaliate against U.S. sanctions imposed last week before the loss of the airliner. It would bar entry to some Americans.
The Ukrainian security council in Kiev said staff of the emergencies ministry had found 186 bodies and had checked some 18 sq.km (7 square miles) of the scattered 25-sq.km crash site. But the workers were not free to conduct a normal investigation.
"The fighters have let the Emergencies Ministry workers in there but they are not allowing them to take anything from the area," security council spokesman Andriy Lysenko said. "The fighters are taking away all that has been found."
Ukrainian Prime Minister Arseny Yatseniuk issued a statement after he spoke to the foreign minister of The Netherlands - more than half those aboard the flight from Amsterdam were Dutch.
He said government experts sent to the site "were not given the opportunity to collect evidence".
"They gave them less than an hour there, and made them leave the site of the catastrophe threatening to take them hostage."
A team of Malaysian experts flew in to Kiev on Saturday and experts from Interpol are due there on Sunday to help with the identification of victims.
Malaysian Transport Minister Liow Tiong Lai said on Saturday he would fly to the Ukraine capital of Kiev to ensure an investigating team gets safe access to the site. Before setting off, he said it would be "inhumane" not to have access, but said Moscow was trying to help: "They are trying their level best to assist Malaysia to ensure we have a safe site," Liow said.
Defence Minister and former transport minister Hishammuddin Hussein said a priority was to ensure debris was not tampered with.
"We want to get to the bottom of this," he added, saying that Malaysia had been in touch with officials in Russia, Ukraine, the United States, Britain and China.
"We do not have a position until the facts have been verified, whether the plane was really brought down, how it was brought down, who brought it down," he said.
As tales of personal grief unfolded, Malaysian Prime Minister Najib Razak revealed his own family was involved - his 83-year-old step-grandmother had been aboard the flight.
The United Nations said 80 children were aboard. The deadliest attack on a commercial airliner, follows the disappearance of flight MH370 in March with 239 passengers.
The scale of the disaster could prove a turning point for international pressure to resolve the crisis in Ukraine, which has killed hundreds since pro-Western protests toppled the Moscow-backed president in Kiev in February and Russia annexed the Crimea peninsula a month later.
"This outrageous event underscores that it is time for peace and security to be restored in Ukraine," Obama said, adding that Russia had failed to use its influence to curb rebel violence.
Some commentators even recalled Germany's sinking of the Atlantic liner Lusitania in 1915, which helped push the United States into World War One, but outrage in the West at Thursday's carnage is not seen as leading to military intervention.
The U.N. Security Council called for a "full, thorough and independent international investigation" into the downing of the plane and "appropriate accountability" for those responsible.