The World Health Organization (WHO) has given the flooding in Pakistan a grade 3 emergency rating, its highest level, AFP reported Wednesday.

“We are releasing $10 million from the WHO Contingency Fund for Emergencies, which is supporting our work to treat the injured, deliver supplies to health facilities, and prevent the spread of infectious diseases,” WHO chief Tedros Adhanom Ghebreyesus said.

Army helicopters flew sorties over cut-off areas in Pakistan's mountainous north Wednesday and rescue parties fanned out across waterlogged plains in the south as misery mounted for millions trapped by the worst floods in the country's history.

Monsoon rains have submerged a third of Pakistan, claiming at least 1,160 lives since June and unleashing powerful floods that have washed away swathes of vital crops and damaged or destroyed more than a million homes.

United Nations chief Antonio Guterres called it "a monsoon on steroids" as he launched an international appeal late Tuesday for $160 million in emergency funding.

Officials say more than 33 million people are affected -- one in every seven Pakistanis -- and it will cost more than $10 billion to rebuild.

The focus for now, however, is reaching tens of thousands still stranded on hills and in valleys in the north, as well as remote villages in the south and west.