Ukraine sought Friday to avoid a political crisis after the shock resignation of its prime minister, as fighting between the army and rebels close to the Malaysian airliner crash site claimed over a dozen more lives.

President Petro Poroshenko called on parliament to heed "cold reason" and pass a vote of confidence in the government, a day after premier Arseniy Yatsenyuk walked out in fury over the collapse of his ruling coalition.

A total of 230,000 people have fled their homes during the spiraling armed conflict in eastern Ukraine, the United Nations refugee agency said Friday.

UNHCR spokesman Dan McNorton told reporters that the number of people who have left the conflict zone for other Ukrainian regions was now close to 100,000, while some 130,000 had crossed the border into Russia.

Three hours after the digging began, a piece of white sackcloth was seen in the soil.

A few minutes later, four bodies were being recovered from the hole.

Australian troops plan to join a police contingent in helping secure the Flight MH17 crash site in Ukraine, Prime Minister Tony Abbott said Friday, while stressing the mission would be humanitarian in nature.

Abbott has been highly critical of the response on the ground to the downing of the Malaysia Airlines plane, which was carrying 298 people when it was apparently shot down in rebel-held eastern Ukraine.

Ukrainian troops have retaken the strategically-important city of Lysychansk in eastern Ukraine, as they press on with their offensive to stamp out a pro-Russian rebellion, President Petro Poroshenko said.

"Ukrainian forces have raised the flag over the town council in Lysychansk," the presidency said in a statement late Thursday.

U.S. President Barack Obama joined with the leaders of Australia and the Netherlands on Thursday to demand full access to the crash site of flight MH17.

Speaking in telephone calls with Australia Prime Minister Tony Abbott and Dutch counterpart Mark Rutte, the leaders agreed to work for a full investigation into the shooting down of the Malaysia Airlines jet last week.

The United States on Thursday said it had evidence Russian forces were firing artillery from inside Russia on Ukrainian troops, in what officials called a "clear escalation" of the conflict.

Moscow is also planning to "deliver heavier and more powerful multiple rocket launchers" to the pro-Russian separatist forces in Ukraine, U.S. deputy State Department spokeswoman Marie Harf said.

Russian President Vladimir Putin's military intervention in Ukraine is fanning nationalist sentiments that could spread across the region with dangerous, unpredictable consequences, the U.S. military's top officer said Thursday.

General Martin Dempsey, the chairman of the U.S. Joint Chiefs of Staff, said Putin was pursuing an "aggressive" agenda that flouts sovereignty and seeks to address alleged grievances harbored by Moscow since the demise of the Soviet Union.

Dutch investigators into the shooting down over Ukraine of flight MH17 said Thursday they had downloaded data from the black box flight recorder and that it had not been tampered with.

"No evidence or indications of manipulation of the recorder was found," the Dutch Safety Board (OVV) said in a statement.

A Canadian warship with a crew of 250 headed Thursday to the Mediterranean where it will join a NATO fleet providing increased security in Eastern Europe amid tensions with Russia over Ukraine.

HMCS Toronto, which is equipped with torpedoes, anti-air and anti-ship missiles as well as a Sea King helicopter, will replace HMCS Regina, which has been part of the NATO operation since May.