Shakhtarsk, Ukraine: New columns of tanks, trucks and heavy artillery have rumbled towards the pro-Moscow rebel stronghold of Donetsk as fears grow of a return to all-out fighting in war-torn eastern Ukraine.

Meanwhile, a Ukrainian separatist leader met senators in Moscow after elections condemned by the US and the European Union, which warned it could increase sanctions on Russia if the conflict intensifies.

Tension mounting: Troop movements have sparked fears of a return to open warfare in the Donetsk region of Ukraine, where Malaysia Airlines flight MH17 was shot down in July. Credit:AFP

Intense weekend shelling around Donetsk and more armoured columns heading to the city have fuelled concerns the rebels could be gearing up for an offensive after weeks of localised skirmishes.

The West has repeatedly expressed concern at Ukrainian claims that Russian military reinforcements are being sent in, while Moscow denies it is involved in the fighting in the east.