A 7-km (4-mile) lake in northern Pakistan, created by a mountain mudslide, is threatening to burst and unleash potentially "catastrophic" floods downstream, officials warned on Saturday.

The mud flow descended into the main Ghizer River channel and blocked it completely on Friday, creating the lake in Gilgit Baltistan province, the National Disaster Management Authority said.

The blockage created a "dam-like structure" that poses a significant threat of bursting, it said in a situation report by its provincial office.

The new lake "can cause a catastrophic flood", said Zakir Hussain, director general of the Provincial Disaster Management Authority.

Four downstream districts - Ghizer, Gilgit, Astore and Diamer - face a serious threat, he told Reuters.

Ghizer is north of the mountain districts in northwest Pakistan where floods triggered by the worst of this year's monsoon rains and cloudbursts have killed nearly 400 people since August 15.

A video shared by the national authority on a WhatsApp group where it issues statements shows black mud sliding down the mountain before landing in the river. Reuters could not independently verify the video, which an official at the authority said was shot by residents.

Similar mud flows landed in the river from different mountainsides, said provincial government spokesperson Faizullah Faraq.

A shepherd on higher ground, the first to spot the mud flow crashing down, alerted villagers and local authorities, he said. As a result of the warning, he said, nearly 200 people in dozens of scattered houses tucked in the mountainsides and the river's surroundings were rescued.

The lake has started discharging water, meaning the threat of a burst is receding, but flash floods in downstream districts cannot be ruled out until the lake is completely cleared, Faraq said.

The communities downstream have been directed to stay on high alert and vacate areas along the river, he said.

Floods across Pakistan have killed 785 since the monsoon started in late June, the national authority said, warning of two more rain spells by September 10.