The United Nations has vowed to help victims of flooding in southern Pakistan, responding to Islamabad's calls for international help for up to five million people affected by recent monsoon rains.

A UN statement issued on Sunday said the humanitarian crisis in Pakistan was "still growing" after exceptionally heavy rainfall.

It said it would quickly begin distributing food, water and housing in Sindh province, which was the hardest-hit area with thousands of houses and large areas of crops destroyed.

Yousuf Raza Gilani, Pakistani prime minister, had appealed for international assistance in a televised address late on Saturday.

"International organisations and the world community should focus their attention on the affected people," Gilani said.

"We are sure that international organisations and the world community would sympathetically consider the appeal by the president of Pakistan and take immediate steps for the rehabilitation of flood-affected people."

President Asif Ali Zardari on Thursday asked the UN to issue an international appeal for humanitarian assistance.

Crisis 'of great magnitude'

The floods have killed 138 people in a month besides affecting millions, Pakistan's National Disaster Management Authority (NDMA) says.

Prime Minister Gilani said the recent rains in Sindh had inundated an area of 16,592 sq km, including 6,879 sq km of crops.

"The situation in Sindh is already serious and there will be more flooding and more problems because of these rains," Arif Mehmood, a meteorological department official, said.

Zafar Qadir,the chairman of the NDMA, said Pakistan was facing a fresh crisis "of great magnitute".

The country is still recovering from last year's floods, which affected 21 million people and killed an estimated 1,750. More than a year later, over 800,000 families remain without permanent shelter.

Aid workers expressed fears over possible outbreak of diseases linked to the new floods, especially among children.

"The biggest issue is that they will drink water from anywhere, so water-borne diseases are a threat, especially diarrhoea and cholera," Sami Malik, a spokesman for the United Nations Children's Fund, told Reuters news agency.

"An adult might not drink water if it's dirty. But a child, if he's thirsty, would just drink it."