DONETSK: International observers turned back Wednesday after making another attempt to reach the site where Malaysia Airlines Flight 17 went down in eastern Ukraine, and a government official said the area near the zone had been mined by pro-Russian separatists who control it.

Observers from the Organization for Security and Cooperation in Europe set out in two vehicles - without frustrated crash investigators from the Netherlands who have been trying to reach the site for four days.

The OSCE observers headed back to the city of Donetsk after discussions with rebels on the city's outskirts not long after starting what would have been a two-hour journey to the site.

That means that almost two weeks after the July 17 disaster, safety concerns and hindrance from the separatists who control the area are still obstructing access to the site. Foreign governments whose citizens died have complained the site is still not secured and some human remains have not been recovered. International observers say wreckage has been cut, moved or otherwise tampered with.

Government security spokesman Andriy Lysenko added to those concerns Wednesday by saying separatists "have mined the approaches to this area. This makes the work of the international experts impossible."

Lysenko was asked at a briefing about concerns that Ukrainian efforts to win back territory were increasing fighting in the area and slowing access. He said that Ukrainian troops weren't conducting operations against separatists near the site, but were trying to cut off their supply lines to force them to leave the area.

OSCE observers did not immediately tell journalists accompanying them what specific issue made them turned back.

The U.S. and Ukrainian governments say the Boeing 777 was shot down by a missile fired from areas controlled by pro-Russian separatists who have been fighting the Ukrainian government. The separatists deny shooting down the plane; Russia denies providing the Buk missile launcher and says the Ukrainian military may have shot the plane down.

Elsewhere, Ukrainian forces took control of the town of Avdeevka, just to the north of the rebel stronghold of Donetsk. The town is near the airport, which has been fought over for weeks by rebels and government forces. Local officials said fighting over the past 24 hours killed 19 people in the region.

Ukrainian forces continue to encircle Horlivka, another key town northeast of Donetsk. The city of Donetsk is one of the main strongholds for the insurrection in the east and taking Horlivka would open the way to move against Donetsk, the Ukrainian military has said.