Ukraine accused Russia and pro-Moscow rebels on Saturday of destroying evidence of "international crimes" as guerrillas and foreign observers faced off over access to the wreckage of the downed Malaysian airliner.
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