The worst could be yet to come for people in flood-ravaged Pakistan, International Development Secretary Andrew Mitchell has said.
Speaking on a visit to the country, Mr Mitchell warned that forecast further heavy rains threatened even more damage to heavily populated areas in the south.
British public donations to the floods appeal hit £19 million on Wednesday, but charities warned that millions were still at risk of cholera and other potentially deadly diseases.
Mr Mitchell and Baroness Warsi, the Minister Without Portfolio, visited Pakistan to see how UK aid was helping victims of the disaster and to assess whether further help was needed.
They spoke of their shock at the scale of the tragedy after touring the village of Pir Sabak in north-west Pakistan, which was destroyed by the raging waters.
Mr Mitchell said: "Nothing could have prepared me for the horrific scenes of destruction and devastation I have seen today.
"But the worst could still be to come. More heavy rains are expected, and as the River Indus rises, it threatens further damage to heavily populated areas in the south of the country."
He added: "We need to support Pakistan for the long-term as well as giving short-term relief and I want the people of Pakistan to know the UK is standing by them."
About 1,500 people have been killed and millions of others displaced by the floods, which have affected a fifth of Pakistan, according to officials.
UK Government funding for victims of the disaster now totals £31 million and is benefiting up to three million people, the Department for International Development said. Another RAF flight carrying UK aid is leaving for Pakistan on Wednesday.