So far, a tonne of explosives has failed to break the ice threatening the bridge, Polish radio reported.

Temperatures are set to plunge again

The local authorities say the bridge - around 60km to the west of Warsaw - is in danger of collapsing unless urgent action is taken.

The helicopters lowered explosives onto the river, in what looks set to become a lengthy operation to save the 1,200m-long structure.

Two ice-cutters are hoping to join the operation if they can reach the area.

Cold snap

The local mayor says a 6km stretch of ice wedged itself under the bridge during the recent unseasonably cold snap.

The several metres thick ice is pressing against the bridge's low supports putting it in danger of collapse.

Officials say the block could also cause flooding in the surrounding area, by acting as an effective dam.

This year's particularly cold winter has caught the Polish authorities off-guard and has already claimed more than 150 lives.