Donetsk, Ukraine (CNN) -- International investigators and observers were again prevented from reaching the crash site of Malaysia Airlines Flight 17 on Tuesday by fierce fighting in the area between pro-Russian rebels and Ukrainian forces.
The Dutch Justice Ministry said the team was unable to leave the city of Donetsk because "there is too much fighting at the moment on and near the route to the disaster site."
The 50-person team of Dutch and Australian experts, accompanied by monitors from the Organization for Security and Co-operation in Europe, was also forced to abandon its attempts to reach the site Sunday and Monday.
The latest setback comes as European Union ambassadors prepare to discuss broader sanctions against Russia over its actions in Ukraine, in line with new measures to be taken by Washington.
U.S. and Ukrainian officials have said that a Russian-made missile system was used to shoot down MH17 from rebel territory. Russia and the rebels have disputed the allegations and blamed Ukraine for the crash.
Twelve days since Flight 17 was blown out of the sky, the Dutch investigators in charge of finding out what happened have yet to lay eyes on the wreckage or the human remains believed still to be strewn across the huge debris field near the town of Torez.
Dutch Prime Minister Mark Rutte asked Ukrainian President Petro Poroshenko in a phone call Tuesday morning to halt the fighting around the crash site so that investigators can access it, Rutte spokesman Jean Fransman said.
On Monday, Ukrainian forces launched an offensive to try to recapture a series of towns on the main road leading toward the debris field.
However, Andriy Lysenko, the spokesman for Ukraine's National Security and Defense Council, told reporters it was the militants' activities that have prevented access to the crash site, not fighting by Ukrainian forces.
He said the rebels would be forced to leave the area. "As soon as they leave, the experts will be able to start working there. But at the moment it's dangerous; we cannot guarantee security on the territories occupied by the terrorists," he said. Ukrainian authorities routinely refer to the rebels as terrorists. "We are doing everything to liberate the territories."
Lysenko also accused the pro-Russian militants of using civilians as human shields as Ukrainian forces try to push them out of their strongholds in Donetsk and Luhansk.