Fierce fighting in eastern Ukraine where a Malaysian airliner was downed further complicated an investigation on Sunday as Europe and the United States prepared economic sanctions on Russia over the conflict, according to The Associated Press.

At least 13 people were killed in clashes between Ukrainian troops and pro-Russian rebels that raged in five areas around the wider region, the AP reported. International monitors said they had abandoned plans to visit the crash site because of fears it was not safe, even though Malaysia said earlier that rebels had agreed to provide access.

Ukraine said it was trying to dislodge the rebels, but denied it was fighting near the crash site, saying the separatists had put the monitors off by falsely claiming that the army was operating nearby, according to the AP.

Russia dismissed U.S. allegations it was about to hand over more missiles to the separatists, who Western leaders say almost certainly shot the airliner down by mistake with a Russian-supplied surface-to-air missile, the AP reported.

The separatists deny any involvement and Moscow says it has not supplied them, suggesting Ukrainian forces were to blame, according to the AP.

"Kiev is trying to destroy the evidence of a crime by its army," separatist leader Aleksander Borodai said, referring to a Ukrainian army offensive some distance from the site on Sunday, the AP reported.

The U.S. State Department sought to bolster the case for robust action by releasing images it said showed Russian forces had fired across the border at the Ukrainian military in the last week, according to the AP.

The Russian Foreign Ministry said Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry had agreed on the need to ensure a swift ceasefire in what it described as an "internal conflict," but the State Department said Kerry did not accept Lavrov's denial that heavy weapons from Russia were contributing to the conflict and urged him "to stop the flow of heavy weapons and rocket and artillery fire from Russia into Ukraine, and to begin to contribute to deescalating the conflict," the AP reported.