With aid agencies struggling to deliver relief supplies, US Secretary of State Hillary Clinton has promised long-term assistance to the Caribbean nation.

After an hour-long meeting with Haiti's President Rene Preval in Port-au-Prince, Clinton told journalists: "As President Barack Obama has said, we will be here today, tomorrow and for the time ahead."

Clinton arrived in Port-au-Prince on Saturday evening after travelling to the devastated capital aboard a US Coast Guard plane.

"I know the resilience and strength of the Haitian people. You have been severely tested," Clinton said. "But I believe that Haiti can come back stronger and better in the future."

The US relief operation has been officially dubbed "Operation Unified Response," General Douglas Fraser, head of the US Southern Command, said on the command's official page on Twitter.

Clinton emphasized that that the U.S. relief operation – involving thousands of soldiers, sailors and Marines along with civilian aid workers -- was done at the invitation of Haiti's government.

Flanked by former presidents Bill Clinton and George W. Bush, US President Barack Obama on Saturday issued a united appeal for Haiti's quake victims.

Sporadic fighting by armed gangs and reports of widespread looting have hampered rescue efforts. A strong aftershock measuring 4.5 on the Richter scale also temporarily halted aid operations as desperate survivors struggled to find food and water.

UN peacekeepers patrolling the capital said popular anger is rising and warned authorities and aid organizations to increase security to guard against looting after Tuesday's earthquake.

Officials estimate 140,000 people were killed in the 7.0 magnitude quake. Around 1.5 million have been left homeless in what the United Nations described as the worst disaster it has ever been confronted with.

UN spokeswoman Elisabeth Byrs said damage to the country's infracture and government losses have made it harder for relief agencies to work than after the Asian tsunami of 2004. "The whole country is in despair and it's overwhelming the international community," said Byrs in a televised interview.

UN suffers losses

United Nations Secretary General Ban Ki-Moon visited Port-au-Prince and toured the ruins of the UN mission on Sunday and then met with Haitian president Rene Preval. Before departing, Ban said the three main priorities in Haiti were to save as many people as possible, to bring emergency humanitarian aid in the form of water, food and medication, and to coordinate the massive aid effort.

The UN announced on Saturday that the chief of its stabilization mission in Haiti and his deputy died in UN's headquarters in Port-au-Prince's Christopher Hotel building.

Ban confirmed that MINUSTAH civilian chief Hedi Annabi, his Brazilian deputy Luiz Carlos da Costa and acting police commissioner, Doug Coates of the Royal Canadian Mounted Police, were confirmed to have died.

"The United Nations was his life and he ranked amongst its most dedicated and committed sons," the UN chief said of Tunisian Annabi. "In every sense of the word they gave their lives for peace," the UN chief added.

The three deaths announced Saturday brought to 40 the number of UN staffers confirmed dead in the quake, with nearly 330 others still unaccounted for.

The United Nations Security Council will convene an emergency session on Monday to discuss the crisis.

The German government has increased emergency aid to Haiti to 7.5 million euros. Meanwhile German Foreign Minister Guido Westerwelle has confirmed the death of a German national. Another 30 Germans are still missing.

nrt/dpa/AFP/Reuters

Editor: Andy Valvur