The deadly attack took place early on Friday, less than 20km from the crash site, which has been the object of sustained battles for several days and still remains in rebel hands.
Ukraine's security spokesman, Vladislav Seleznev, slammed the rebels for violating a UN brokered "day of quiet," calling for a halt in fighting in the area.
"The militants are behaving in a cowardly and shameless fashion," Seleznev said.
"The used the 'day of quiet' just to fire on us."
Defence officials said another 13 servicemen remained unaccounted for after the attack.
Fighting in the area had previously hindered international investigators from finding and retrieving bodies that have been decaying in the summer heat.
On Friday, a team of several dozen international investigators finally descended on the crash site, now designated as a crime scene.
Investigators from the Netherlands, Australia and officials from the OSCE will focus on recovering several dozen bodies from the wreckage and also aim to retrieve the belongings of the 298 people killed when the Boeing 777 was shot down last month.
It is believed up to 80 bodies may still remain uncollected at the crash site, which is sprawled in a broad area across fields between two villages.
Despite both sides in the ongoing conflict in east Ukraine tentatively agreeing to a ceasefire around the crash zone, fighting is continuing nearby.