Islamabad: Pakistan will have to demonstrate it can spend relief funds transparently and well if it wants more help in rebuilding after its massive floods, the US aid chief said, as the UN appealed urgently for more helicopters to ferry aid to around 800,000 stranded people.

America has been the most generous contributor to the flood aid, rushing in emergency assistance to support a vital ally in the war against Al Qaida and Taliban. But rebuilding homes, roads, livelihoods and vital infrastructure will cost billions of dollars, and there are questions over who will pay.

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, which has a reputation for inefficiency and corruption.

Rajiv Shah, administrator of the US Agency for International Development, said the US 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 AP on Tuesday.

The floods began almost a month ago with the onset of the monsoon and have ravaged a massive swath of the country, from the mountainous north through to its agricultural heartland. More than 8 million people are in need of emergency assistance.

Some of the routes along which trucks carry supplies to US and Nato troops in Afghanistan have also been affected by the floods. A spokesman for international forces in Afghanistan said supplies had been slowed down but there had been no impact on operations.