The British public is "shaming politicians across the world" with the generosity of its response to the devastating floods in Pakistan, the Disasters Emergency Committee said today.

The DEC– the umbrella organisation for British aid agencies – said the crisis caused by monsoon rain in the last three weeks combined the scale of the Asian tsunami and the destruction of the earthquake in Haiti.

Its chief executive, Brendan Gormley, said the British public had donated £30m, with contributions rising during the appeal's second week, About £6m came in over the weekend. "The UK public are leading the way and shaming politicians across the world," he said. "We want the rest of the international community to back that sustained giving."

Aid workers say it could be at least two or three years before many of those in the most severely affected areas in Pakistan could see their lives rebuilt. Reports from the country suggest aid is still not getting through. UN aid officials said it had taken time for the international community to realise how bad the situation was.

Pakistan estimates that it could cost £10bn to rebuild devastated parts. Rains that first fell in northern areas such as Swat have caused "walls of water" to sweep south, especially along the Indus river.

Ashraf Mal, Pakistan manager for the Tearfund charity, said that in Sindh, the southern area now most threatened by floods, there was a "huge wall of water. People are fighting a losing battle trying to save their towns, villages and homes."

A senior UN official earlier criticised the "quite extraordinary" lack of international support for Pakistan. Louis-Georges Arsenault, the director of emergency services for Unicef, said the flooding was the biggest humanitarian crisis in decades.

"One of the major challenges that we have, which is quite extraordinary, is the lack of level of support from the international community," he said. "Right now, our level of needs in terms of funding is huge compared to what we've been receiving – even though this is the largest, by far, humanitarian crisis we've seen in decades."

The International Monetary Fund is preparing to meet Pakistani officials in Washington to discuss measures to stave off an economic crisis after the devastation.

Tens of thousands of people are trying to flee the latest flood surge in southern Pakistan, three weeks after huge monsoon rainfall hit the country. About 1,600 people are thought to have died and an estimated 20 million have been affected by the disaster, 10 million having lost homes, livestock or crops.

The World Health Organisation has warned that diseases are spreading, with hundreds of hospitals and clinics damaged or destroyed. Jane Cocking, the humanitarian director of Oxfam, said casualties could rise sharply if there was a significant outbreak of waterborne disease and those affected were unable to get swift medical care.

The UN says it has so far raised about 70% of the $460m (£295m) it had set as an initial aid target. Britain has earmarked up to £64.3m, with £16.8m already committed to providing tents, blankets, food supplements for babies and other relief. A separate £10m bridge-building project has also been brought forward. The DEC money is not included in this sum.

It is the first time in the DEC's 45-year history that donations have risen rather than fallen during the second week of an appeal. Donations to the appeal, launched on 5 August, increased by 18% in the second week and have not yet started dropping.

"It is sometimes harder to fully appreciate the impact of disasters, such as floods, that take longer to develop," Gormley said. "However, the response of the UK public to the floods in Pakistan has been extraordinary. We have never seen anything like it. The full impact of the disaster will only be revealed when flood waters finally recede and the extent of devastation is uncovered."