Pakistanis line up during aid distribution by the United Nations World Food Program at a camp for families displaced by floods in Sukkar, Sindh province, southern Pakistan, on Saturday. (Kevin Frayer/Associated Press)

The flood surge in Pakistan is moving south, forcing thousands of people in Sindh and Balochistan provinces to leave their homes and move to higher ground.

Floods triggered by monsoon rains have killed more than 1,500 people in Pakistan and left at least six million people in need of urgent assistance, the United Nations says.

"We have hundreds of thousands of people on the move," UN spokesman Maurizio Giuliano said in a statement Monday.

"The floods are outrunning our relief efforts. We move faster and faster, but the finish line keeps moving further ahead."

The latest town under threat from the Indus River is Shadad Kot in the hard-hit southern Sindh province. On the eastern side of the city, the levee was under pressure from waters three metres high, said Yaseen Shar, the top administrative official there.

"We are fighting this constant threat by filling the breaches with stones and sandbags but it is a very challenging task," he said.

Most of the city's residents have moved to relief camps or to towns and cities away from the danger that has damaged almost half a million homes in Sindh province alone.

Many people fleeing flooding in Sindh have moved to higher areas in Balochistan, where the UN refugee agency has established five camps to house people in need of shelter.

The flood waters have devastated lives from the mountainous north to the southern plains, and they are expected to begin draining into the Arabian Sea in the coming days — but officials have said relief efforts still need to be scaled up.

More helicopters needed

Local charities, the Pakistani army and international agencies are providing food, water, medicine and shelter to the displaced, but millions have received little or no help. Aid officials warn that water-borne diseases like cholera now pose a real threat.

Many communities have been cut off by the floods, forcing officials to rely on boats and helicopters to evacuate people and deliver aid.

The World Food Program said three more helicopters arrived in Pakistan over the weekend to help deliver emergency relief, but the agency said at least 40 more helicopters are needed to deliver supplies to those who need it.

On Sunday, the Pakistani government said the world has given or pledged more than $800 million in aid to the country.

Canada has pledged $33 million and on Sunday said it would match, dollar for dollar, money raised by Canadians for relief operations.

The money will go to the newly established Pakistan Floods Relief Fund with equal contributions made to Canadian registered charities between Aug. 2 and Sept. 12, said government House leader John Baird on Sunday.

"We have not placed a dollar limit on the amount of funds that we will provide through this important program," he said.

With files from The Associated Press