qPort-au-Prince, Jan 13 (DPA) A magnitude 7 earthquake struck Tuesday evening near this capital of Haiti, blocking roads, collapsing buildings and leaving bodies strewn in the streets.

With spotty communications into the stricken country and Haiti's weak government itself hard hit, international donors were Wednesday organising and the US was mounting rescue and relief efforts.

The quake struck at 4.53 p.m. (2153 GMT Tuesday), some 15 km south-west of the city at a depth of 10 km. Two aftershocks registering 5.9 and 5.5 on the Richter scale followed within the hour, with more temblors later.

'Many buildings have collapsed. Many people are looking for their relatives,' Pastor Vasquez, spokesman for the Dominican embassy in Port-au-Prince, told DPA.

'There is big, very big chaos. Sirens are heard, people are running desperately.'

After the initial quake, witnesses described a white, ghostly cloud that filled Port-au-Prince streets, a sign of the scale of devastation from collapsed concrete buildings. The dust lingered in the air until darkness fell soon after.

US Secretary of State Hillary Clinton called it a 'catastrophic earthquake'.

With the city in darkness, the death toll and extent of damage were difficult to determine. The US deputy chief of mission in Port-au-Prince, David Lindwall, reported to Washington that the embassy had not been damaged but that there were numerous fatalities in the city.

'Clearly, there's going to be serious loss of life in this,' State Department spokesman Philip Crowley said.

'One of the questions will be, at first light, what's the condition of the airport, and at what point can we begin to put teams on airplanes and get them down there?'

Crowley said that US relief teams being assembled in Virginia and California would work closely with the US Agency for International Development. A task force was 'working on this issue through the night', he said.

Crowley said communications within Haiti were difficult because landlines and cellphone towers had likely been disturbed by the quake. He said embassy officials had not yet been able to determine the safety of all employees.

Haitian President Rene Preval and his wife were unharmed, Raymond Alcide Joseph, Haitian ambassador to the US, told CNN. But the presidential palace and Commerce Ministry were damaged.

'If those buildings are damaged, can you imagine what has happened to all those flimsy abodes around Port au Prince on the hillsides?' Joseph asked.

He recalled his growing dismay over the unregulated building of shacks on the hillsides, and how he had written an article some years ago saying it was a 'catastrophe waiting to happen'.

'The National Palace and the ministries of finance, labour, communications and culture suffered serious damage,' Haitian television channel Haitipal reported.

One hospital collapsed in the Petion Ville residential neighbourhood on the outskirts of Port-au-Prince.

Peacekeeping authorities at UN headquarters in New York said 'a large number of personnel remain unaccounted for'.

'Contacts with the UN on the ground have been severely hampered as communications networks in Haiti have been disabled by the earthquake,' said Alain LeRoy, UN undersecretary general for peacekeeping operations.

He said the headquarters of the UN Stabilization Mission in Haiti in Port-au-Prince and other UN installations had sustained serious damage. The mission has more than 7,000 military and civilian personnel deployed in Haiti to assist the government in implementing programmes on development and democratic reform.

UN Secretary General Ban Ki-moon expressed concern for the people of Haiti and UN staff serving there, and said he was following developments closely.

'My heart goes out to the people of Haiti after this devastating earthquake,' he said.

With a population of 9 million, Haiti is the poorest country in the western hemisphere and has been plagued by political turmoil for decades. The UN extended its 7,000-member mission in the impoverished country late last year.

In Washington, US President Barack Obama immediately offered help.

'My thoughts and prayers go out to those who have been affected by this earthquake,' Obama said. 'We are closely monitoring the situation, and we stand ready to assist the people of Haiti.'

Inter-American Development Bank president Luis Alberto Moreno said the Washington-based lender would immediately approve an emergency assistance grant of $200,000 for Haiti 'to provide food, potable water, medicines and temporary shelter to victims'.

'We are monitoring these events very closely and stand ready to help Haiti address this catastrophe,' he said. 'In coordination with the Haitian government, we will redirect our uncommitted portfolio to provide additional reconstruction resources at this moment of dire need. We are working with other donors to exchange information and

coordinate response activities.'

The extent of casualties and damage were not immediately clear.