With the sound of artillery blasts at a distance, 70 international investigators arrived Friday at the eastern Ukraine site where Malaysia Airlines Flight MH17 crashed and started recovering the remains of as many as 80 victims that have been lying in farm fields for two weeks.

Several hours before they arrived, at least 10 Ukrainian soldiers were killed when their convoy was ambushed by pro-Russian separatist rebels in a town close to the wreckage site. Thirteen more soldiers were unaccounted for after the attack, officials said, and the bodies of four more people were being examined to determine whether they were soldiers or rebels.

But the team was able to launch what is expected to be a painstaking search and "brought back all the human remains they found," said Ilona de Ruyter, a spokeswoman for the Dutch-led recovery operation. She provided no further details on the remains.

The investigators from the Netherlands and Australia, plus officials with the Organization for Security and Co-operation in Europe, travelled from the rebel-held city of Donetsk in 15 cars and a bus to the crash site outside the village of Hrabove.

As they set up a base to work from at a chicken farm, an Associated Press reporter heard artillery fire in the distance. It was impossible to tell how far away shells were landing and whether the Ukrainian army or rebel forces were firing.

The investigative team's top priority is to recover human remains that have been rotting in midsummer heat of 32 C since the plane went down on July 17. They will also try to retrieve the belongings of the 298 people killed aboard the Boeing 777.

Gloves and blue plastic buckets

After they arrived, members of the team wearing gloves broke up into small groups and walked into fields of scrub. They placed items into blue plastic buckets, but it was impossible for journalists prevented from getting close to see what they had collected.

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Despite both sides in the ongoing conflict in east Ukraine tentatively agreeing to a ceasefire around the MH17 crash zone, fighting is continuing nearby. (Maks Levin/Reuters)

Other team members gathered around plane wreckage, taking photos of debris from the jet's fuselage and tail. Rebel fighters guarding the perimeter of the zone stayed away from the investigators and some patrolled the streets of the neighbouring village of Rozspyne.

Ukraine and the West contend the plane was shot down by the rebels with a Russian-supplied missile. Rebel leaders publicly deny it, but one top rebel official has told the AP on condition of anonymity that insurgents were involved in the operation that downed the plane.

Friday's search effort came after a smaller advance investigative team managed to perform a preliminary survey of the area a day earlier. For days, clashes along routes to the wreckage site had kept investigators from reaching the site. Independent observers warned that there has been tampering with evidence.

The sprawling site of fields in between two villages is now designated a crime scene and was being divided into grids for systematic searches for remains, belongings and jet crash evidence, Australian police officer Brian McDonald told reporters in Hrabove. Specially trained dogs will be also be used in the search, McDonald said.

The investigative team's journey in their convoy lasted about three hours from Donetsk, through the government-held town of Debaltseve, and back into the separatist-controlled territory, where the wreckage lies. At Debaltseve, the convoy was joined by three vehicles from the International Committee of the Red Cross.

Town in rebel hands

Both sides in the conflict tentatively agreed to a ceasefire around the crash zone, but the Friday morning attack by rebels on government troops took place less than 20 kilometres south of the site, outside the town of Shakhtarsk. Ukrainian forces and rebels have been battling in that area for several days but the town is still in rebel hands.

Ukraine crisis

Ukrainian forces suffered heavy losses overnight after a rebel ambush killed at least 10. (Valentyn Ogirenko/Reuters)

Defence officials said that an army convoy was struck by mortars during redeployment.

Ukraine security spokesman Vladislav Seleznev said the attack took place at 6 a.m. local time, before the end of the 24-hour "day of quiet" declared Thursday in response to a call for a cease-fire from UN Secretary General Ban Ki-moon.

"The militants are behaving in a cowardly and shameless fashion," Seleznev said. "They used the 'day of quiet' just to fire on us."

Another Ukraine defence spokesman, Andriy Lysenko, said another 13 soldiers have been reported missing in action after the attack.

Seleznev said the bodies of another four people killed in the incident have not yet been identified.

Ukrainian forces in attempting to regain control of the area are trying to drive a wedge into an area between the largest rebel-controlled cities, Donetsk and Luhansk. Shakhtarsk lies on one of two highways linking those cities.

In Donetsk, meanwhile, one person was killed Friday and three others were wounded when mortar fire struck a minibus carrying passengers near the central train station, said city hall spokesman Maxim Rovensky.

The city government in Luhansk said five residents were killed and nine wounded after artillery shells rained down on them.

Officials in the two cities did not say who they believe was responsible for the shelling.

Russian helicopters in Ukraine airspace: official

Ukraine has routinely accused Russia of supplying rebels with heavy weaponry and firing rockets onto Ukraine from its territory.

Ukraine

Ukrainian government artillery guns stand in a field near the village of Debaltseve, Ukraine. (Dmitry Lovetsky/Associated Press)

On Friday, Lysenko said that Russian military helicopters repeatedly crossed between 10 and 15 kilometres into Ukrainian airspace a day earlier.

Lysenko said the Ukrainian anti-aircraft defence system reported another three incursions by aircraft flying at 800 kilometres per hour.

The goal of the aircraft, said Lysenko, was to maintain pressure on Ukrainian armed forces.

Ukraine's Foreign Ministry said in a statement that Russian troops lobbed shells across border at Ukrainian troops on at least four separate occasions on Wednesday and Thursday.

The statement added that a lengthy convoy of armoured vehicles and trucks carrying fighters had crossed the border into Russia.

Kyiv's accusation that it has been subject to rocket attacks from Russia have typically been dismissed out of hand by Moscow, but Ukraine's security services attempted to bolster its claims this week by presenting what it said was photographic evidence.

On Wednesday, the head of the security services Valentyn Nalyvaichenko said Ukrainian satellite imagery showed burn marks from rockets recently fired at Ukrainian troops from a position two kilometres inside Russian territory.

Similar claims were supported by separate satellite data released by the U.S. State Department.