(BBC) - Heavy artillery fire has rocked the east Ukrainian city of Donetsk, the industrial hub held by pro-Russian separatist rebels.

Shelling can been heard in the city almost daily but Wednesday's bombardment was the strongest since the weekend, AFP news agency reports.

It was unclear whether the fire came from besieging government forces or the rebels themselves or both.

There are also reports of fighting near the rebel-held city of Luhansk.

In another development, Nato's top commander, US Gen Philip Breedlove, accused Russia on Tuesday of moving forces "capable of being nuclear'' to Crimea, the Ukrainian territory it annexed in March after a controversial referendum.

Nato did not know if nuclear weapons were actually in place, the Associated Press news agency reports.

One Ukrainian soldier was killed and another injured north of Luhansk, when rebels fired on government positions near the village of Schastya, Ukrainian security forces said.

More than 4,000 people have died since government forces moved in April to put down an armed insurrection by the rebels in the two regions, which border Russia.

A fragile ceasefire has been in place since 5 September, although hundreds of people have been killed since then.