Last Updated Jul 28, 2014 4:15 PM EDT
KIEV, Ukraine -- Heavy fighting raged Monday around the Malaysia Airlines Flight 17 debris field, once again preventing an international police team charged with securing the site from even getting there.
At least eight civilians have been killed by fighting and shelling in two Ukrainian cities held by separatist militants, officials in the rebellion-wracked east said Monday.
Authorities in Luhansk that five people were killed and 15 injured by overnight artillery strikes. Three were killed in Donetsk as a result of clashes, the city's government said.
Territory between the cities has seen intensified fighting as government troops try to gain control over the area where Malaysia Airlines Flight 17 was downed earlier this month. The pro-Russia separatists have been blamed for shooting down Flight 17 with a surface-to-air missile on July 17, killing all 298 people aboard.
Andriy Lysenko, spokesman for Ukraine's national security council, denied that Ukrainian forces are fighting within the 12-mile radius around the crash site where President Petro Poroshenko last week said there would be a cease-fire to allow investigators to work.
"Ukrainian forces are not conducting combat operations in the area where the plane crashed," he told a briefing. "There is a ban on this." He said, however, that rebels were shelling the area.
While Ukrainian officials deny fighting around the crash site, U.S. officials tell CBS News the Ukrainian Army is fighting well, and appears to be winning back the area.
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.
Unreleased data from a black box retrieved from the Boeing 777's wreckage shows findings consistent with the plane's fuselage being hit multiple times by shrapnel from a missile explosion.
"It did what it was designed to do," a European air safety official told CBS News late last week, "bring down airplanes." The official said the data extracted showed a "massive explosive decompression" of the plane's cabin.
Dutch and Australian police set off for the crash site Monday morning in a convoy of 20 cars, aiming to secure the area so that investigations can continue and any remaining bodies can be recovered. They didn't get far. The convoy stopped in Shakhtarsk, a town around 20 miles from the fields where the aircraft was downed. Sounds of regular shelling could be heard from Shakhtarsk and residents were fleeing the town in cars. AP reporters saw a high-rise apartment block in the town being hit by at least two rounds of artillery.
As CBS News correspondent Charlie D'Agata reports, it was the second day in a row that a contingent of international police were forced to abandon their attempt to reach the crash site due to the heavy fighting.
The Australian and Dutch police in Ukraine are not armed, but they have been eager to reach the crash area in hopes of enabling investigators to get on with the now extremely delayed work of documenting the wreckage and the remains of dozens of bodies still believed to be entangled in the debris.
Ukraine's foreign minister says pro-Russian separatists are continuing to try to manipulate the wreckage of the Malaysian airliner that the United States and others have accused the rebels of shooting down.
In an interview with The Associated Press, Foreign Minister Pavlo Klimkin said new fighting that prevented an international police team from getting to the site earlier Monday was the fault of the separatists who are "trying to wipe out any sort of traces" of their involvement. He said a cease-fire in the area of the crash site remains a priority for authorities in Kiev.
Both sides in the conflict have traded accusations over the mounting civilian death toll. The armed conflict that has been raging for more than three months has displaced more than 200,000 people.
Rebels accuse government troops of deploying artillery against residential areas. Authorities deny that charge, but also complain of insurgents using apartment blocks as firing positions.
The U.N. High Commissioner for Human Rights said in a report out Monday 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 U.N. report from mid-June put the death toll at 356.
Ukraine's National Security Council has said government forces have surrounded Horlivka, a city about 20 miles north of Donetsk, and D'Agata reports that fighting has also been reported in the town of Torez, the nearest town to the crash site.
While the Ukrainian military is trying to regain control of the region, D'Agata says in terms of the investigation, it's just making the situation worse.
As the government intensified its offensive, Foreign Minister Klimkin told CBS' "Face the Nation" on Sunday that his government was eager for a bilateral cease-fire.
"We are for (a) peaceful settlement," Klimkin said. "But we can't negotiate directly with terrorists who shot down the plane, who have been killing people, taking hostages...The main leaders of the terrorists are Russian citizens with different connections with Russian special services."
The U.S. State Department on Sunday released satellite images that it says back up its claims that rockets have been fired from Russia into eastern Ukraine and heavy artillery for separatists has also crossed the border.
Russian Foreign Minister Sergey Lavrov dismissed the claims Monday during a televised press conference, asking "why it took ten days" before the U.S. released the images.
A four-page document released by the State Department appears to show blast marks from where rockets were launched and craters where they landed. Officials said the images, sourced from the U.S. Director of National Intelligence, show heavy weapons fired between July 21 and July 26 - after the July 17 downing of Malaysia Airlines Flight 17.
The images could not be independently verified by The Associated Press.
Lavrov said he is expecting OSCE observers to arrive at the Russian-Ukrainian border "in the coming days." He said they would see that accusations rebels are traveling freely into Ukraine from Russia are false.
Ukrainian officials have said the mission is largely pointless because it involves only about two dozen observers monitoring the 1,240-mile border between the two countries.
Meanwhile, U.S. officials told CBS News correspondent David Martin that Russia continues to reinforce troops along the border, although the exact number of reinforcements beyond the 15,000 reported last week is unclear. Additionally, military equipment - artillery and vehicles - continue to flow into the Rostov supply base near the border.
Most of the Russian troops are within 10 miles of the border and they are more heavily armed than the 40,000 man buildup of several months ago, which also was further back from the border. That gives them a greater capability and flexibility to intervene quickly - as they have been doing with artillery and potentially to move into eastern Ukraine.