At least 10 government troops have been killed in an ambush by pro-Russian separatist rebels in an area near the crash site of Malaysia Airlines flight MH17, Ukrainian authorities said today.

Defence officials said in a statement that the attack happened during redeployment in the town of Shakhtarsk, which has been at the centre of sustained battles for several days.

Ukrainian forces have latterly focused their strategy on driving a wedge into an area between the two main rebel strongholds of Donetsk and Luhansk.

Shakhtarsk lies on one of two highways linking those cities and is about 20km south of where the Malaysia Airlines plane came down.

The Boeing 777 was brought down in a missile attack last month, killing all 298 people on board.

Meanwhile, a convoy of international investigators set out for a second visit to the crash site but on a road that did not go through Shakhtarsk.

A small team managed to perform an initial survey of the area for the first time yesterday.

For days, clashes along routes to the wreckage site had kept investigators from reaching the area to find and retrieve bodies that have been decaying in the 32C midsummer heat. Independent observers warned that there has been tampering with evidence.

Today, the team were travelling in 15 cars and one bus from their base in the rebel stronghold of Donetsk. Their convoy was joined by three vehicles from the International Committee of the Red Cross when it reached the government-controlled town of Debaltseve.

Both sides in the conflict in east Ukraine have tentatively agreed to a ceasefire around the crash zone, although there is evidence that fighting is continuing in nearby locations.

It is believed up to 80 bodies may still remain uncollected at the crash site, which is spread in a broad area across fields between two villages.

Investigators working on the site say their first priority will be to recover human remains and retrieve victims' belongings, so that they can be returned home.