A powerful 7.3-magnitude earthquake has struck off the coast of Haiti, rocking the capital and sparking a tsunami alert for neighbouring states.

Haiti's envoy to the US said it was a "catastrophe of major proportions" as reports from the capital spoke of casualties and damage.

The tremor hit at 1653 (2153 GMT), about 15km (10m) south-west of Port-au-Prince, the US Geological Survey said.

A hospital and other buildings are reported to have collapsed.

A visiting US official told AP the sky in the city was "just grey with dust" and he could hear distant screaming. The quake was quickly followed by two strong aftershocks of 5.9 and 5.5 magnitude.

The US Pacific Tsunami Warning Center said there was the possibility of a local tsunami that could affect "coasts located usually no more than a 100km [60 miles] from the earthquake epicentre".

A tsunami watch was in effect for Haiti, the neighbouring Dominican Republic, Cuba and the Bahamas.

The BBC's Nick Davies in neighbouring Jamaica says the ground apparently shook for more than a minute in Haiti.

Local people, he said, were using anything they could get their hands on - including farm equipment - to help release those trapped in the quake.

Our correspondent adds that, as the poorest country in the western hemisphere, Haiti is likely to need international aid in order to cope with the quake's impact.

'Rubble and wire'

Raymond Joseph, Haiti's ambassador to the US, told CNN: "I think it is really a catastrophe of major proportions."

He said he had just spoken to a government colleague in Port-au-Prince.

"He had to stop his car just about half an hour ago, and take to the streets, start walking, but he said houses were crumbling on the right side of the street and the left side of the street," the ambassador said.

"He does not know whether he would reach his home, not knowing what he would find, because he had a bridge to cross to get there."

An AP cameraman saw the wrecked hospital in Petionville, a hilly suburb of the capital, and Henry Bahn, a visiting official from the US Department of Agriculture, said he had seen houses which had tumbled into a ravine.

"Everybody is just totally, totally freaked out and shaken," said Mr Bahn.

He had, he continued, been walking to his hotel room when the ground began to shake.

"I just held on and bounced across the wall," he said.

"I just hear a tremendous amount of noise and shouting and screaming in the distance."

Rocks, he added, were strewn all over the place, and the ravine where several homes had fallen in was "just full of collapsed walls and rubble and barbed wire".