Ukraine said its troops had wrested more territory from pro-Russian rebels on Monday and were advancing towards the site where Malaysian flight MH17 was brought down, which international investigators said they could not reach because of the fighting, according to Reuters.

Ukrainian officials said troops had recaptured two rebel-held towns near the crash site and were trying to take the village of Snezhnoye, near where Kiev and Washington say rebels fired the surface-to-air missile that shot down the airliner with loss of all 298 on board, Reuters reported.

One pro-government militia said 23 of its men had been killed in fighting in the past 24 hours, according to Reuters.

Analysis of black box flight recorders from the airliner showed it was destroyed by shrapnel from a missile blast which caused a "massive explosive decompression", a Ukrainian official said on Monday, Reuters reported.

Investigators in Britain, who downloaded the data, had no comment, according to Reuters. They said they had passed information to the international crash investigation led by the Netherlands, whose nationals accounted for two-thirds of the victims.

In a report on three months of fighting between government forces and separatist rebels who have set up pro-Russian "republics" in the east, the United Nations said more than 1,100 people had been killed, Reuters reported.

U.N. High Commissioner for Human Rights Navi Pillay said increasingly intense fighting in the Donetsk and Luhansk regions was extremely alarming and the shooting down of the Malaysian airliner on July 17 may amount to a war crime, according to Reuters.

Western leaders say rebels almost certainly shot the airliner down by mistake with a Russian-supplied surface-to-air missile, but Russia accuses Kiev of responsibility, Reuters reported.

The separatists are still in control of the area where the plane was shot down but fighting in the surrounding countryside has been heavy as government forces try to drive them out, according to Reuters.

On Monday at least three civilians were reported killed in overnight fighting, and Kiev said its troops recaptured Savur Mogila, a strategic piece of high ground about 20 miles from where the Malaysia Airlines Boeing hit the ground, and other areas under rebel control, Reuters reported.