At least nine people died when floodwaters swept away children in a river in northern Pakistan and relatives jumped into the water to try to save them on Friday, officials said.

The family was having a picnic breakfast by the Swat River and the children were in the water taking photos when the sudden flood hit, district administrator Shehzad Mahboob said.

Relatives rushed in but were also caught up in the deluge which was swollen by monsoon rains, he added.

It was still too early to say how many children and how many adults had died, Mahboob told Reuters. Nine bodies have been recovered so far, he said. Four members of the family were rescued alive and another four are still missing.

A group of people were seen standing on a small patch of ground surrounded by fast-moving water in an online video that Reuters verified was shot on Friday on the Swat River.

Locals and more than 80 rescue workers were searching for survivors, rescue official Shah Fahad said.

The Provincial Disaster Management Authority later issued an alert saying there were high flood levels and warning people to take precautions.

The family group were tourists from Pakistan visiting the Swat Valley, local mayor Shahid Ali Khan said.

Tens of thousands of tourists, mostly from other parts of Pakistan, visit the north's peaks and glaciers every year during the summer travel season.

Prime Minister Shehbaz Sharif "expressed his grief over the tourists' deaths," his office said in a statement.