Haitians marked a sad anniversary Monday, mourning the hundreds of thousands killed six months ago in a massive earthquake and eyeing a precarious future amid the slow trickle of aid.

In the ruins of the presidential palace toppled by the January 12 quake that measured 7.0 in magnitude, international supporters joined Haitian President Rene Preval and other leaders in a poignant remembrance ceremony.

"We are going to continue to help the people in the camps, but today we want to officially launch the reconstruction phase," said Preval, stressing the difficulties of even clearing away the debris to start rebuilding.

"We need some 1.5 billion dollars to get rid of 20 million cubic meters of rubble. Before we can rebuild, we need to clean up," he added.

Former US president Bill Clinton, who is heading up the UN effort, promised greater transparency in using the funds sent to help rebuild the impoverished Caribbean nation. And he praised the work that had been done so far.

Some 250,000 people were killed and 1.5 million left homeless when the earth briefly convulsed on January 12, unleashing a trail of destruction on the capital, Port-au-Prince.

Tent cities have sprung up in and around the ruined capital of the poorest country in the Americas, with little sign that those left homeless and destitute will move into more permanent housing any time soon.

Prime Minister Jean-Max Bellerive called for better coordination in the reconstruction phase. "The NGOs need to sit down with the government to draw up and carry out, one single plan for the reconstruction of Haiti," he said.

Three months after an international conference in New York where world powers pledged more than 10 billion dollars over five years, only a fraction of the promised aid has materialized.

The US Center for Strategic and International Studies calculated Monday that just two percent or about 50 million dollars had actually arrived.

Private donations raised some 1.6 billion dollars for non-governmental organizations, but only 20 to 40 percent of the funds they have received have been disbursed, the CSIS said.

"In spite of this outpouring of generosity, the challenge of aid delivery remains a central issue," it wrote.

The United States vowed however it still stood by Haiti.

"Six months later, our resolve to stand with the people of Haiti for the long term remains undiminished," said Secretary of State Hillary Clinton.

Cheryl Mills, a senior adviser to Clinton, said there have been successes in Haiti: no major disease outbreak, improved access to banking and -- unlike after the 2004 Indian Ocean tsunami -- little local inflation as aid pours in.

Interior Minister Paul-Antoine Bien-aime used Monday's remembrance to thank the international community for its help, saying the "situation would have been much worse" without its assistance.

But Haitians are growing impatient at the slow trickle of aid and the crawling pace of reconstruction, with experts predicting it could take 20 years just to clear the rubble from the streets.

"We have moved 250,000 cubic meters of rubble, which sounds like a lot, until you realize there's 20 million cubic meters of rubble here," UN spokeswoman Imogen Wells told CNN.

USAID chief Rajiv Shah said the amount of rubble still lying on the streets "is probably more than 20 times that which existed in other tragedies such as the World Trade Center" destroyed in the September 11, 2001 attacks.

Shah said one of the challenges was teaching local builders to make stronger cement "so that bricks stick together," adding some rebuilt homes in the Delmas slum were now two to three times stronger than before the quake.

According to the United Nations office in Haiti, nearly 4,000 small homes have been built in a project that anticipates building some 10,000 houses.

And the French Red Cross, which has vowed to build some 30,000 temporary homes with the American Red Cross, has launched construction of the first 500 outside the capital.

But with hurricane season already underway, the UN has warned that another tragedy may be looming.

It has identified 130 tent cities as at risk from rains and winds that could further worsen conditions for the most vulnerable -- including thousands of orphaned children.