Invesigators were prevented from accessing the crash site of Malaysia Airlines Flight 17 for a second day in a row as the fighting between pro-Russia rebels and the Ukrainian military continued in east Ukraine.

Pro-Russia separatists are still in control near the crash site, raising concerns as to whether investigators will be able to collect evidence from the crash before it is destroyed completely. Investigators from the Netherlands, Australia and Malaysia returned to the site, which is in Donetsk, to collect evidence, The Washington Post reports.

Yet, on Monday, Ukrainian forces appeared to be gaining ground, as the military reported that they overtook the rebel area of Saur Mogila. The military said it was a major gain, as it blocks a supply route that they claim Russian rebels are using to supply the Ukrainian separatists with ammunition and artillery. The blockade of the route also allows the Ukrainian military to resupply its troops while gaining mountaintop positions for the military.

The military said their main objective at the moment is to make the rebels vacate the area of the crash.

As the fighting continues, debris, and possibly human remains, are at the crash site, which is frustrating international observers.

"There's a job to be done," Alexander Hug, the deputy head of a monitoring team from the Organization for Security and Cooperation in Europe, told the Associated Press. "We are sick and tired of being interrupted by gunfights, despite the fact that we have agreed that there should be a cease-fire."

On Monday, U.N. high commissioner for human rights Navi Pillay also said that the shooting down of the plane by a Russian missile "may amount to a war crime".

A senior White House official also announced Monday that the U.S. will levy more sanctions against Russia, and the E.U. will follow suit this week.

President Barack Obama spoke with leaders from Germany, Britain, France and Italy to discuss the impending sanctions.

"We expect the European Union to take significant additional steps this week, including in key sectors of the Russian economy," deputy national security advise Antony Blinken told The Washington Post. "In turn, and in full coordination with Europe, the United States will implement additional measures itself."

Blinken said that Russian shipments of additional weapons have been sent to the Ukrainian rebels, which bolsters evidence that "[Russia] is firing from positions inside of Russia into Ukraine."

"And we've seen a significant re-buildup of Russian forces along the border, potentially positioning Russia for a so-called humanitarian or peacekeeping intervention in Ukraine," he added.

He said that Russia is working to "destabilize the country," but that the Ukrainian government has made "major gains." He said Russia was using the attention from the crash to their own advantage.

European Union leaders agreed on a preliminary draft to impose sanctions against major sectors in the Russian economy. However, it is not yet clear if all of the 28 member states of the E.U. will agree to the new round of sanctions.

This month, the U.S. already imposed sanctions on Russian banks, as well as on defense and energy companies.

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