Islamabad, Pakistan (CNN) -- Heavy downpours worsened massive flooding in Pakistan on Sunday as aid agencies increased relief efforts to help the about 12 million people affected in much of the country.
The flooding, which started in the northwest, threatened places as far south as the port city of Karachi.
Peter Kessler, a spokesman, for the United Nations High Commissioner for Refugees, called the situation "catastrophic."
"There are whole families just sitting and camping in the road median strips, people just lined along the streets anywhere they can get safely while there is a very real sense of nervousness that worse may be coming," Kessler said.
Districts in the southern province of Sindh were on high alert as more rain swelled bloated rivers and inundated drenched earth.
Sindh authorities have evacuated at least 150,000 people from low-lying areas, according to a statement from the United Nations on Saturday.
"The water levels are very high, and the risk of serious flooding is increasing rapidly," said Andro Shilakadze, head of the United Nations Children's Fund's office in Sindh. "We are planning for the worst-case scenario, so that we can provide emergency relief as may become necessary."
The U.N. said needs throughout the country were on par with those that arose after the 2005 earthquake affected more than 3 million people and caused widespread damage.
Pakistan's Federal Flood Commission warned Saturday that the Indus River was flooded and rising in parts of Sindh, including Sukkur, the third largest provincial province.
As many as 12 million people have been affected by the torrential rains and floods, Pakistani authorities said. About 1,600 people have died.
Pakistan's Meteorological Department said more heavy rains were in store in the next few days, threatening to deepen the worst natural disaster the nation has experienced in recent history. And the monsoon season is only halfway over.
The nation's disaster management authority estimated that 650,000 houses have been damaged or destroyed. By comparison, the earthquake of 2005 affected 3.2 million people and damaged or destroyed 611,000 homes.
Up to 500,000 people were homeless in Punjab province, the U.N. said. At least 1.4 million acres of farmland were destroyed in the province, where people rely heavily on agriculture for food.
"Pakistan has been hit by the worst flood of its history," Prime Minister Yousuf Raza Gilani said in a televised speech Friday. "As I speak, the flood is still engulfing new areas and adding to the scale of devastation."
Gilani said a true assessment of the loss to life and infrastructure can be determined only after the water recedes. He appealed to the international community and to Pakistanis living abroad to help.
As the rains continued, aid agencies scrambled to help those in need.
"The assistance that we have so far provided has alleviated suffering, but relief operations need to be massively scaled up," said Martin Mogwanja, the U.N.'s humanitarian coordinator in Pakistan.
Kessler said their warehouses were covered in more than 3 meters of water for days. Most of the aid in the warehouses, which are the size of Walmart supercenters, is damaged and destroyed.
He said they're hoping to salvage plastic sheeting quickly, and added that the tents are wet and need to be dried before use.
U.N. officials said the government is cooperating and granting visas and permits to get teams and aid into the country.
Parts of the Swat Valley remain inaccessible by road; helicopters provide the only way in or out for people, food, clean water and medicine. Bad weather hampered the flights Saturday.
Meanwhile, victims accused the government of lack of action. They criticized President Asif Ali Zardari, who was in England for talks with British Prime Minister David Cameron during the worst of the crisis.
"We elected him," a resident of the flooded town of Chachran Sharif said. "Where is he?"
U.S. emergency relief teams continued to arrive in Peshawar to help, the U.S. State Department said.
In all, the U.S. Agency for International Development has committed $35 million in aid to international organizations and nongovernmental organizations, it said.
CNN's Reza Sayah, Brian Walker and Moni Basu contributed to this report.