DONETSK, July 27 - 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, Reuters reported.

At least thirteen people were killed in clashes between Ukrainian troops and pro-Russian rebels which raged in five areas around the wider region.

International monitors said they had abandoned plans to visit the crash site due to fears it was not safe, even though Malaysia said earlier 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.

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