At least 20 people including six civilians have been killed during heavy fighting overnight Thursday in eastern Ukraine, authorities said on Friday.

According to Ukrainian military operation spokesman Oleksiy Dmytrashkivsky, 14 people died in clashes between government troops and insurgents near the Shakhtarsk town, one of spots that witnessed violent confrontation over the past days.

Ten of those killed were Ukrainian servicemen, while identities of the other four victims remained unknown, Dmytrashkivsky told reporters.

In Donetsk city, at least one person was killed and four others were injured, when an explosive device went off inside a bus.

In the neighboring Lugansk city, five civilians were killed and nine others wounded, when shelling hit a supermarket and apartment blocks.

Despite fighting in the eastern regions, an international investigation team reached the crash site of Malaysia Airlines Flight MH17 for the second consecutive day.

According to the Organization for Security and Cooperation in Europe (OSCE), Dutch and Australian experts and police officers, as well as monitors from the OSCE were continuing their work near the village of Grabovo, where a Boeing 777 went down two weeks ago, killing all 298 people on board.

On Thursday, the international experts, who had failed in three attempts to reach the crash site, had found access to the area and started the search of victims' bodies.