Tourists who braved record low temperatures to visit Niagara Falls have been treated to a spectacular sight.

Water turned to ice as it plunged over the 34-metre high American Falls - one of three waterfalls which form the popular tourist attraction on the border between the United States and Canada.

Icy chunks tumbled over the crest of the falls and long icicles were left dangling above the frozen Niagara River below.

The unusual sight came as a polar vortex that gripped the US for four days, plunging temperatures to -37C (-35F) and leaving at least 21 people dead, finally began to ease its grip.

The crippling cold, which affected more than half the US population as it spread from the Midwest to the east and south, has been pushed northwards, leaving warmer weather and calmer winds in its wake.