Relief workers in Pakistan are struggling to reach hundreds of thousands of people who have been cut off by rising flood waters, the United Nations says.

The floods, caused by heavy monsoon rains, have washed out key bridges and roads in many parts of South Asian nation.

As many as 800,000 people in need of assistance can only be reached by air, the United Nations said.

Access has been particularly difficult in the Swat Valley of the northwestern Khyber Pakhtunkhwa Province and in the mountainous areas of Gilgit- Baltista, UN officials said.

Conrad Sauvé, the secretary general of the Canadian Red Cross, said reaching people in remote areas has been a major challenge.

He said relief workers are working alongside the government to deliver aid to camps and hard-to-reach communities, but he warned that the crisis is still unfolding.

"This is still an emergency situation, and unfortunately, with the flooding, new people arrive all the time because new areas are flooded," Sauvé told CBC News from Islamabad.

"When they think they have something under control, there's new people that are arriving, so it's a major challenge."

The World Food Program has said at least 40 more helicopters are needed to reach the people cut off by the floods.

Meanwhile, the flood waters sweeping down the Indus River in Pakistan are threatening new areas in Sindh province, forcing thousands of people to head for higher ground.

Rising waters have already poured through several communities in the southern province, and officials are now urging some 200,000 people to move out of the Thatta area of Sindh province, the BBC reported.

Saifullah Dharejo, the irrigation minister for Sindh province, said high tides were preventing the bloated Indus River from quickly emptying into the Arabian Sea as had been hoped.

The UN says more than 1.2 million homes have been damaged or destroyed since the flooding began. Roughly one million people have received tents and tarps, but an estimated five million people still need emergency shelter, the UN said.

At least 1,500 peole have died since the unprecedented floods began in in the north in late July.

Pakistani government must be accountable: USAID chief

The Pakistan government says about $800 million in emergency aid has been committed or pledged so far. But there are concerns internationally about how the money will be spent by the government.

A boy sits in front of houses destroyed by floodwater in Muzaffargarh district of Punjab province August 25, 2010. (Asim Tanveer/Reuters)

Rajiv Shah, administrator of the U.S. Agency for International Development (USAID), said the United States would continue to urge nations to donate.

"We are going to work at it, but these are tough economic times around the world and it will require a demonstration of real transparency and accountability and that resources spent in Pakistan get results," he said in an interview with The Associated Press late Tuesday.

Along with the government, local and international agencies and the U.S. military, a number of Islamist groups have been providing aid to flood victims. At least one of the groups is alleged to be a front for Lashkar-e-Taiba, a banned militant organization blamed for the 2008 attacks in Mumbai, India.

The government in the northwest issued an order Wednesday barring banned Islamist groups from operating relief camps, said Mian Iftikhar Hussain, the chief spokesman for the province.

But it was unclear whether any such camps had been shut down by Wednesday evening or how exactly the government was going about identifying them.

With files from The Associated Press