The United Nations refugee agency rushed in more desperately needed aid on Monday to flood-stricken Pakistan as the nation's prime minister travelled to the south, where rising waters of Lake Manchar pose a new threat.

Two UNHCR planes touched down in the southern port city of Karachi, and two more were expected later in the day. A third plane, with aid from Turkmenistan, also landed in Karachi. While the floods in recent weeks have touched much of Pakistan, the southern Sindh province, where Karachi is the capital, has been the most affected.

More than 1,300 people have been killed and millions have lost their homes in flooding caused by unusually heavy monsoon rains in Pakistan this year that many experts have blamed on climate change.

In response to the unfolding disaster, UN Secretary General Antonio Guterres last week called on the world to stop "sleepwalking" through the crisis. He plans to visit flood-hit areas on Friday.

On Sunday, engineers cut into an embankment in the sides of Lake Manchar in an effort to release rising floodwater to save the city of Shewan and several nearby villages from possible destruction by flooding waters, which have damaged 1.6 million houses since mid-June.

Prime Minister Shehbaz Sharif was met by Foreign Minister Bilawal Bhutto Zardari in the city of Sukkur on the swelling Indus River, from where they toured the flood-hit areas by helicopter. Murad Ali Shah, the province's chief minister, briefed Sharif about the damage caused by floods in Sindh.