Pakistan on Thursday asked the United Nations to issue an international appeal for humanitarian assistance for up to five million people affected by the recent monsoon rains, an official said.

"President Asif Zardari talked to UN Secretary General Ban Ki-Moon by telephone today and requested him to issue an international appeal for assistance to the flood-affected people," presidential spokesman Farhatullah Babar told AFP.

"Our foreign office is sending a formal letter to the United Nations in this regard," Babar said.

He said Zardari visited his native Nawabshah and other flood-affected towns to review the relief work before speaking from Karachi with the UN chief.

Monsoon rains in Pakistan have killed 138 people in a month and affected up to five million more, according to the National Disaster Management Authority.

Southern Sindh, a flood-prone province where authorities said more heavy rains were expected in the coming days, was the hardest-hit area, with crops and many houses destroyed.

Last year, the worst floods in Pakistan's history affected 21 million people and killed an estimated 1,750, with the south again taking the brunt of mother nature.

The meteorological office has forecast more rain this week.