New Hampshire became the sixth US state to legalise gay marriage after Governor John Lynch signed the measure into law, even though he says he personally opposes the practice.

At least four people were missing after deadly flooding tore up highways, swept away houses and tossed vehicles around like toys in New Hampshire, authorities said.

Prolonged, heavy rain caused flooding from North Carolina to New Hampshire over the weekend, forcing hundreds of people to evacuate, knocking out electricity, weakening dams and making roads impassable.