Renewed fighting near the crash site of Malaysia Airlines Flight 17 forced Dutch and Australian investigators to cancel plans to visit the wreckage-strewn area Sunday, further delaying efforts to collect evidence and recover remaining bodies.

Ukrainian officials said their forces were attacking separatists on several other fronts in an apparent effort to cut off the city of Donetsk from the other rebel stronghold, Luhansk, and supply routes across the Russian border.

The military, they added, also was trying to retake the area around the crash site to ensure an investigation into the downing of the plane. Both Ukraine and U.S. officials say they have mounting evidence pro-Russia separatists shot down Flight 17 with a surface-to-air missile system likely supplied by Moscow. Rebel officials, who deny shooting down the plane and point the finger at Ukrainian troops, meanwhile accused Kiev of trying to seize the crash site to tamper with evidence.

Authorities investigating the July 17 incident that killed all 298 people aboard, nearly two-thirds of them Dutch, have been frustrated by the lack of full access to the site due to the conflict between Ukrainian government forces and pro-Russia rebels led by Alexander Borodai. The mission's aim is to locate the last remains of passengers and forensically examine the site. But the longer it takes to secure the area, the harder it will be for investigators to gather untainted evidence that could determine who or what brought the plane down.

"If there is no security, the experts can't do their job," said Alexander Hug, head of the monitoring mission for the Organization for Security and Cooperation in Europe.

Heavy fighting was reported around the town of Horlivka, where regional officials reported as many as a dozen civilians, including some children, were killed in a rocket attack. The rebels and the military each blamed the other for the attack. By evening, military officials said Horlivka was surrounded, though that couldn't be independently confirmed.

OSCE spokesman Michael Bociurkiw said a group of the Dutch and the Australian unarmed policemen and experts would try to get to the crash site Monday despite the raging conflict. But a top leader of the pro-Russian insurgents in Donetsk said that the experts may not be able to get to the crash site "for the time being" because of the fighting.

Ukrainian troops "moved there without any tactical reason" said Andrei Purgin, first deputy prime minister of the self-proclaimed Donetsk People's Republic. He added the government forces are "deliberately shelling a patch of land on which we put our hopes to find out where the missile was probably launched from."

Earlier Sunday, Malaysia said an international police force from the Netherlands, Australia and Malaysia--the three countries that suffered the most casualties in the downing of Flight 17--would be deployed to secure the crash site after reaching a deal with the leader of the Ukrainian separatists.

Under the agreement, Malaysia would send 68 police personnel Wednesday as part of the international group, Malaysian Prime Minister Najib Razak said.

Australia is sending unarmed police to the crash site as part of a Dutch-led mission to recover the remaining bodies of passengers, Australian Prime Minister Tony Abbott said. Australia has about 170 federal police stationed in Ukraine ready to assist in securing the site if needed.

"Our objective is to get in, get cracking and get out," Mr. Abbott said. "This is a risky mission, no doubt about that, but all of the professional advice that I have is that the safest way to conduct it is unarmed as part of a police-led humanitarian mission," he told journalists in Canberra.

U.S. and Ukrainian officials accuse Russian-backed separatists of downing Flight 17 during a routine flight from Amsterdam to Kuala Lumpur with a sophisticated surface-to-air missile. Russia has denied any involvement and condemned the attack, suggesting it was carried out by Ukrainian forces.

The crash site's location in rebel-held territory in Ukraine has complicated efforts to collect the remains of the dead and hampered investigation efforts. Dutch forensics experts trying to recover bodies at the crash site were stopped by pro-Russia rebels Saturday before they could reach the area.

The head of the general staff of the separatists in eastern Ukraine, who only gave his nom de guerre as Mykhailo, said the rebel leadership had limited reporters' access to conflict zones but denied it advised the Dutch against going to the crash site.

He added that the separatists had enough power and resources to prevent the Ukrainian army from retaking Donetsk.

Malaysia said the probe would require more than 30 investigators to cover the wide area, but continuing fighting prevented deployment of such a large contingent.

"I hope that this agreement with Mr. Borodai will ensure security on the ground, so the international investigators can conduct their work," Mr. Najib said Sunday.

Mr. Abbott said that the police would stay as long as they needed to bring the remains home but that the operation wasn't expected to take longer than two to three weeks.

"This is a volatile situation. This is contested ground, and we don't want to be there any longer than is absolutely necessary," Mr. Abbott said.

Maarten van Tartwijk contributed to this article.

Write to Jason Ng at jason.ng@wsj.com and Rebecca Thurlow at rebecca.thurlow@wsj.com

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