The scale of the humanitarian crisis in Pakistan is unprecedented, with a third of the country under water, the UN said Friday.

More than 33 million people, 15 percent of the total Pakistani population, have been affected, Dr Palitha Mahipala, the World Health Organization (WHO) representative in the country, said. "More than 6.4 million people are in dire need of humanitarian aid."

In the past few weeks, record monsoon rains dumped more than five times the 30-year average for rainfall in some provinces, killing more than 1,200 people and injuring over 6,000 since June.

As rains continue and flooding is likely to worsen over the coming days, there is an urgent need to scale up disease surveillance, restore damaged health facilities, and ensure sufficient medicines and health supplies to affected communities.

Read: Pakistan fatal flooding has hallmarks of warming

"Affected people have told our staff on the ground about their traumatic and scarring experiences as rain and floodwaters swept away their possessions in minutes," Matthew Saltmarsh, spokesperson for the UN refugee agency, said.

Torrential monsoon rainfall has caused the River Indus to overflow, submerging land for tens of kilometres wide, according to recent images from the European Space Agency.

Crops and livestock have been lost, having a significant impact on both livelihood and nutrition of afflicted communities.

Pakistan, which is already facing political and economic turmoil, has been thrown into the front line of the human-induced climate crisis. The South Asian country of 220 million people faced dramatic weather conditions this year, from record heatwaves to deadly floods.

Pakistan is home to more glaciers than anywhere outside the polar areas. Global warming makes the country more vulnerable to sudden outbursts of melting glacier water, according to the Meteorological Department in Islamabad.