Panicky residents in an eastern Ukrainian town fled their homes Monday carrying a few possessions in plastic bags and small suitcases as shells exploded in the distance, fighting that also prevented an international police team from reaching the area where the Malaysia Airlines plane was downed.

"Mom, hang in there," exclaimed a weeping woman who was fleeing Shakhtarsk with her mother. Associated Press reporters saw a high-rise apartment block in the town being hit by at least two rounds of artillery.

The fighting there and elsewhere in the area kept Dutch and Australian police for the second day from reaching the site where the plane crashed after being shot from the sky. They had planned to begin searching for remaining bodies and gathering forensic evidence and the delay strained tempers among international observers.

"There is a job to be done," said Alexander Hug, the deputy head of a monitoring team from the Organization for Security and Cooperation in Europe. "We are sick and tired of being interrupted by gunfights, despite the fact that we have agreed that there should be a ceasefire."

A Ukrainian official said Monday that data from the recovered flight recorders show that Malaysia Airlines Flight 17 crashed due to a massive, explosive loss of pressure after being punctured multiple times by shrapnel. Andriy Lysenko, spokesman for Ukraine's national security council, said the plane suffered "massive explosive decompression" after it was hit by fragments he said came from a missile.

Rebels may be losing ground

There were signs that government forces were gaining some ground in their fight with the rebels.

Ukraine's Defense Ministry said Ukrainian troops entered Shakhtarsk, although checkpoints blocking the western entrance into town remain under rebel control. It also said fighting was taking place in Snizhne, which lies directly south of the crash site, and in other towns in the east.

Meanwhile rebels in Donetsk said on Twitter that fighting took place in the village of Rozsypne, where some of the wreckage still lays strewn and uncollected.

A rebel military leader, Igor Ivanov, told Russian state news agency RIA-Novosti that the village had fallen into government hands, but that information could not immediately be confirmed.

And in a possible indication of sagging morale within the rebels' ranks, the deputy leader of the rebels in Donetsk announced Monday that his immediate superior, Alexander Borodai, had left for Russia. Viktor Antyufeyev, who is a Russian national like Borodai, said he will take over as the separatist government's acting prime minister.

UN outlines rising death toll

The rising loss of life was underlined by a report released Monday by the United Nations.

The UN High Commissioner for Human Rights said that at least 1,129 people have been killed between mid-April, when fighting began, and July 26. The report said at least 3,442 people had been wounded and more than 100,000 people had left their homes. A UN report from mid-June put the death toll at 356.

The UN said rebel groups continue to "abduct, detain, torture and execute people kept as hostages in order to intimidate" the population in the east. It said rule of law had collapsed in the rebel-held areas and that 812 people had been abducted in the Donetsk and Luhansk regions since mid-April.

With allegations that Russia is supplying weapons to the rebels and allowing Russian fighters to cross into Ukraine, the OSCE said it plans to begin deploying a border-observation mission on the Russian side on Tuesday. The mission is to be deployed at two checkpoints. It is unclear if they will be able to assess whether the border is being crossed at areas without checkpoints.

U.S., EU to impose new sanctions on Russia

The U.S. and EU plan to impose new sanctions against Russia this week, including penalties targeting key sectors of the Russian economy. 

The show of Western solidarity comes as the U.S. accuses Russia of ramping up its troop presence on its border with Ukraine. 

President Barack Obama and the leaders of Britain, Germany, France and Italy discussed the crisis during a rare joint video teleconference on Monday. The discussion follows days of bilateral talks on how to implement tougher sanctions after the downing of the passenger jet. 

The U.S. and European sanctions are likely to target Russia's energy, arms and financial sectors. The EU is also weighing the prospect of levying penalties on individuals close to Russian President Vladimir Putin, who appears to only be deepening Russia's role in destabilizing Ukraine.