Donations from the British public to help flood victims in Pakistan have reached £29 million, according to the Disasters Emergency Committee (DEC).
Chief executive Brendan Gormley said it was the first time that donations had risen in the second week of an appeal.
"We want the rest of the international community to back that sustained giving," he said.
The worst floods to hit the country in decades have left more than 1,500 people dead and an estimated 20 million homeless.
An appeal to help the stricken country was launched two and a half weeks ago by DEC, an umbrella organisation representing 13 leading UK aid agencies.
It is the first time in the DEC's 45-year history that donations rose rather than fell during the second week of an appeal.
Mr Gormley said: "This is a tragedy unfolding in slow motion with new areas still being flooded and the threat of deadly waterborne disasters stalking millions of survivors.
"The full impact of the disaster will only be revealed when flood waters finally recede and the extent of devastation is uncovered."
The death toll was said to have risen to 1,600 on Monday but Jane Cocking, humanitarian director of Oxfam, said it could rise sharply if there is a significant outbreak of waterborne disease and those affected are unable to get swift medical care.
Priorities were clean water, sanitation, food and medical care, she said.
Ms Cocking, who visited Pakistan last week, choked back tears as she described how one "tough" old man broke down when he was asked how he planned to get through the winter.
The scenes were the worst she had seen in more than 20 years in humanitarian work, including 13 years with Oxfam.
Mr Gormley said the scale of the disaster was "beyond our worst nightmares" but UK charities were well placed to help because they have worked in Pakistan for many years.
"UK charities are leading the response on the ground," he said.
DEC member agencies and their partners have so far helped more than 800,000 people.
To make a donation people can call the 24-hour hotline on 0370 60 60 900, visit http://www.dec.org.uk or donate over the counter at any post office or high street bank, or send a cheque.
They can also donate £5 by texting the word GIVE to 70707.
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