Ukrainian army tanks were reported to be launching an assault to break pro-Russian rebels' hold on the eastern city of Donetsk on Monday in the first major outbreak of hostilities in the area since Malaysia Airlines flight MH17 was shot down last week.
A separatist leader said Ukrainian government forces were trying to break into Donetsk and fighting was under way near the railway station.
Sergei Kavtaradze, of the rebels' self-proclaimed Donetsk People's Republic, said at least four tanks and armoured vehicles were trying to break through into the city.
A Ukrainian military spokesman said the operation was in progress but would not comment on reports of troops entering Donetsk. "The active phase of the anti-terrorist operation is continuing. We are not about to announce any troop movements," Vladyslav Seleznyov said.
Reuters journalists also saw two rebel tanks heading towards Donetsk railway station.
As international horror deepened over the fate of the remains of the 298 victims of the air disaster, the first international investigators reached rebel-held territory in eastern Ukraine on Monday.
Three members of a Dutch disaster victims identification team arrived in Donetsk and were expected to visit a railway station near the crash site where nearly 200 bodies have been stored in refrigerated wagons.
Rescuers said they had found a total of 251 bodies and 86 body fragments at the crash site and a second refrigerated train had arrived.
The shooting down of the airliner on Thursday has sharply deepened the Ukrainian crisis, in which separatists in the Russian-speaking east have been fighting government forces since protesters in Kiev forced out a pro-Moscow president and Russia annexed Crimea in March.
The United States and its allies have pointed the finger at the pro-Russian rebels and at Moscow itself over the downing of the plane although Russia has denied involvement.
Shock turns to anger
U.S. Secretary of State John Kerry laid out what he called overwhelming evidence of Russian complicity in the shooting down of the Malaysia Airlines plane.
Kerry demanded that Moscow take responsibility for actions of pro-Russian rebels in eastern Ukraine whom Washington suspects of downing the jet with a missile, and expressed disgust at their "grotesque" mishandling of the bodies.
Television images of the rebel-controlled crash sites, where the remains of victims had lain decomposing in fields among their personal belongings, have turned initial shock and sorrow after Thursday's disaster into anger.
Emotions ran high in the Netherlands, the home country of about two thirds of the 298 people who died in the Boeing 777. The Dutch foreign minister has said the nation is "furious" to hear bodies were being "dragged around", while relatives and church leaders demanded they be rapidly returned home.
But the departure of dozens of corpses loaded into the refrigerated railway wagons was delayed on Sunday as Ukrainian officials and rebels traded blame over why the train had not yet left the war zone, and where or when international investigators would be able to check it.
The U.N. Security Council is scheduled to vote on Monday on a resolution that would condemn the downing of the plane, and demands that those responsible be held accountable and that armed groups not compromise the integrity of the crash site.
In an apparent attempt at compromise with Moscow, the wording of the resolution, drafted by Australia, was changed to characterise the incident as the "downing" of the flight, instead of "shooting down", according to the final draft obtained by Reuters. Diplomats said it was unclear if Russia would support the final version.
In Washington, Kerry criticised Russian President Vladimir Putin and threatened "additional steps" against Moscow.
"Drunken separatists have been piling bodies into trucks and removing them from the site," he said on NBC television on Sunday. "What's happening is really grotesque and it is contrary to everything President Putin and Russia said they would do."
Russia has blamed the Ukrainian military for the disaster. But while stopping short of directly blaming Moscow, Kerry put forward the most detailed U.S. accusations so far, based on the latest U.S. intelligence assessments.
Russian President Vladimir Putin makes a televised statement at the Novo-Ogaryovo state residence outside Moscow, in the early hours of July 21, 2014. Photo credit: Reuters
British Foreign Secretary Philip Hammond urged Moscow to ensure international investigators had access to the crash sites. "Russia risks becoming a pariah state if it does not behave properly," he told Sky television.
Australian Prime Minister Tony Abbott said he had spoken to Putin for the first time about the disaster. At least 27 Australian passengers were on the flight from Amsterdam to Kuala Lumpur.
Abbott said an Australian investigation team was in Kiev but had been unable to travel to the site. He said there had been some improvement with the Ukrainian government offering access.
"But there's still a hell of a long way to go before anyone could be satisfied with the way that site is being treated," Abbott said. "It's more like a garden cleanup than a forensic investigation. This is completely unacceptable."
After lying for two days in the summer heat, the bodies had been removed from much of the crash site by Sunday, leaving only bloodstained military stretchers along the side of the road.
As Ukraine accused the rebels of hiding evidence relating to the loss of the airliner, a separatist leader said items thought to be the stricken jet's "black boxes" were now in rebel hands.
Investigators from the U.N. aviation agency arrived in Ukraine to help to investigate the crash, but a senior official said safety concerns prevented them from reaching the crash site.
Kerry said the United States had seen supplies moving into Ukraine from Russia in the last month, including a 150-vehicle convoy of armoured personnel carriers, tanks and rocket launchers given to the separatists.
It had also intercepted conversations about the transfer to separatists of the Russian radar-guided SA-11 missile system, which it blames for the Boeing 777's destruction. "It's pretty clear that this is a system that was transferred from Russia," Kerry said in an interview on CNN.
Kerry's evidence of a Russian connection tracked closely an official unclassified U.S. intelligence summary released over the weekend. It said intelligence analysts confirmed the authenticity of an audiotape conversation provided to the media by Ukrainian authorities of a known separatist leader boasting of downing the plane.
"We also have information indicating that Russia is providing training to separatist fighters at a facility in southwest Russia" that includes missile systems, it said.
The United States has already imposed sanctions on individuals and businesses close to Putin but Kerry indicated that President Barack Obama might go further. "The president is prepared to take additional steps," he told Fox News, although he ruled out sending in U.S. troops.
European Union ministers should be ready to announce a fresh round of sanctions at a meeting of the EU's Foreign Affairs Council this week, said a statement from British Prime Minister David Cameron's office, issued after telephone calls with French President Francois Hollander and German Chancellor Angela Merkel.