Obama challenges Putin: "What are they trying to hide?"

WASHINGTON (Reuters) - US President Barack Obama increased pressure on Russian President Vladimir Putin on Monday to force pro-Russian separatists to lift their blockade of the international investigation into the downing of a passenger plane last week.

In a statement at the White House, Obama said that Russian-backed separatists in eastern Ukraine continue to keep investigators away from the site where the Malaysia Airlines plane crashed, sometimes firing shots into the air.

"What are they trying to hide?" Obama asked.

Obama said that Putin has a direct responsibility in getting the separatists to cooperate with the investigations and that it is now up to Moscow to insist that the separatists stop obstructing the investigation.

(Reporting by Steve Holland, Jeff Mason and Roberta Rampton)

Fighting continues in Ukraine even with the arrival of investigators.

By Anton Zverev and Peter Graff

DONETSK, Ukraine (Reuters) - Fighting broke out on Monday in the eastern Ukrainian city of Donetsk, while in a nearby region investigators began inspecting the bodies of victims of Malaysia Airlines flight MH17, which was shot down in mid-air last week.

The fighting in Donetsk served as a reminder of the dangers specialists face when working in a war zone. International inspectors gained access to the remains of hundreds of victims stored in refrigerated train cars near the crash site, but governments expressed concerns about the lack of greater access to the area where the plane crashed, which is controlled by rebels.

The Ukrainian government denied sending the regular army to the center of Donetsk, captured by pro-Russian separatists in April, but said that "self-organized" pro-Ukraine groups were fighting rebels in the city.

Four people were killed in clashes near the train station and close to the airport on the outskirts of Donetsk, medical officials said.

Artillery fire sent plumes of smoke billowing up near the Donetsk train station, about 60 kilometers from the plane crash site, in what separatists said was an attempt by government forces to enter the city.

Donetsk is at the center of the rebellion against the central government in Kyiv. Ukrainian President Petro Poroshenko has vowed to retake the city as part of what his government calls an "anti-terrorism operation" against the separatists.

Against the backdrop of international perplexity over the fate of the remains of the 298 victims of the disaster, the first international investigators arrived in eastern Ukraine on Monday.

Three members of a Dutch disaster victim identification team arrived at the train station near the crash site, where rebels said they had stored 247 bodies in refrigerated wagons. About two-thirds of the victims of the tragedy were Dutch.

Ukrainian authorities said on Monday that 272 bodies and 66 body fragments had been found.

"The bodies were stored in good conditions," said Peter van Vliet, whose team entered the train cars wearing surgical masks and rubber gloves.

Ukrainian Deputy Prime Minister Volodymyr Groysman said on Monday that all the bodies would be transported by refrigerated train to the eastern city of Kharkiv, with departures expected after 19 p.m. (14 p.m. Brasília time). Afterwards, all the bodies would be taken to the Netherlands, according to Groysman.

According to Ukrainian authorities, 272 bodies and 66 body fragments had been found.

(Additional reporting by Pavel Polityuk)