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The UN is set to hold an emergency session to boost international aid to flood-hit Pakistan.

It has raised nearly half the $460m (£295m) wanted for initial relief but says the response remains slow.

The number of people in need of immediate assistance in Pakistan has now risen to eight million, with more than half of them without shelter.

And there are fears of new flooding, as water continues to surge south along the Indus River.

UN Secretary General Ban Ki-moon called the special session after visiting Pakistan last week to assess the disaster.

He described the situation as "heart-wrenching" and said he had never seen a disaster on such a scale.

The member states are expected to adopt a resolution urging the international community "to extend full support and assistance" to Pakistan in its efforts "to mitigate the adverse impacts of the floods and to meet the medium- and long-term rehabilitation and reconstruction needs".

The ����ý's Kim Ghattas at the UN says the resolution will not produce any concrete action plan but is a sign of how nervous the US and the UN are about the level of international assistance given to Pakistan so far.

US Secretary of State Hillary Clinton is expected to announce an increase in US donations, said White House officials.

The session is a clear attempt to build a sense of urgency about a natural disaster that will have a lasting impact on a country that is key to the fight against the Taliban and al-Qaeda, says our correspondent.

'Pretty scary'

A UN spokesman said on Wednesday that there had been an improvement in the speed of donations from the international community, after a sluggish response in the first days of the appeal.

"Donors are realising the scale of the disaster," Maurizio Giuliano told Reuters, "but the challenges are absolutely massive and the floods are not over."

"The size of this disaster is equivalent to Austria, Switzerland and Belgium combined. That's pretty scary."

On Wednesday, the European Union promised an extra $39m following higher commitments from Australia and Japan, while the Islamic Development Bank pledged $11.2m.

But fewer than one million of the eight million people the UN says are in urgent need have received basic supplies such as tents or plastic sheeting.

Some 4.6 million people were without shelter, spokesman Maurizio Giuliano said on Thursday, most of them in the provinces of Punjab and Sindh.

Some of them would be able to return to homes they were forced to leave, he said, but many would have to rebuild houses shattered by the force of the floods.

Pakistani authorities say as many as 20 million people are affected by the floods. Tens of thousands of villages remain under water.

In a new aid move, the Asia Development Bank (ADB) said it had offered to loan Pakistan $2bn to help it recover.

The loan would help Pakistan rebuild infrastructure damaged by the worst ever flooding in the country, the bank's director-general for central and west Asia, Juan Miranda, told the Financial Times.

"We have a long-term commitment to the country. This is a time when we have to show what we're made of, to work with everyone to figure out exactly how we can put back dignity into the lives of the people."

He said the bank would also set up a trust fund to channel donor contributions for reconstruction.

The World Bank has already agreed to lend Pakistan $900m to help long-term reconstruction.

Officials and aid agencies in Pakistan say there are signs that the crisis is still deepening, as new floodwaters continue to surge south along the Indus river and more flood defences collapse, forcing people to flee their homes.

There are growing health concerns for those surviving without proper shelter, food or clean drinking water, three weeks after the country's worst natural disaster began