Islamabad, Pakistan (CNN) -- Some 14 million people have been affected by the torrential rains and flooding that have left more than 1,000 dead in Pakistan, a spokesman for the national disaster authority told CNN Tuesday.
While 1,245 people have been confirmed killed, officials believe the number of dead may reach 1,500, said spokesman Ahmed Kamal.
He said 1,334 people have been injured and 337,282 people have been rescued. More than 302,000 houses have been damaged, Kamal said, but that number might more than double by the time the flooding subsides.
Most of the displaced have sought shelter in public buildings, including schools, said a spokeswoman for the United Nations High Commissioner for Refugees. They include thousands of Afghan refugees "who have suddenly lost their homes for the second time."
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. The monsoon season is only halfway over.
The flooding, which started in the northwest, threatened places as far south as the port city of Karachi.
Nineteen people died across the country between Sunday and Monday nights, Kamal said.
The weather in the northwest province of Khyber Pakhtunkhwa did improve slightly Tuesday, which allowed 23 helicopters to take off for relief and rescue operations, according to a military spokesperson who did not want to be identified.
Kamal said 76 helicopters and 621 boats were participating in rescue efforts.
Of those affected by the flooding, 6 million to 7 million are in immediate need of food and other items, said Maurizio Giuliano, information officer for the United Nations Office for the Coordination of Humanitarian Affairs.
"While not all may be in need of immediate humanitarian assistance, and the severity of their needs has not yet been fully assessed, this is a higher figure than those who were affected by the 2005 South Asia tsunami (3 million), the 2005 South Asia earthquake (3 million), or the 2010 Haiti earthquake (3 million)," he said Monday in a statement.
Though the number of fatalities was not comparable with either the tsunami or the earthquake, both of which had death tolls that exceeded 200,000, Pakistan's flooding "is a major disaster of enormous magnitude," said John Holmes, U.N. Under-Secretary-General for Humanitarian Affairs and emergency relief coordinator.
Many Pakistanis are not happy with the government's response to the floods, saying it has been slow and ineffective.
Pakistani President Asif Ali Zardari returned home Monday from an overseas trip, facing growing criticism of his actions following the massive flooding. He had been in England for talks with British Prime Minister David Cameron during the worst of the crisis.
Prime Minister Yousaf Raza Gilani said a true assessment of the loss of life and infrastructure can be determined only after the water recedes. He appealed to the international community and to Pakistanis living abroad to help.
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. humanitarian coordinator in Pakistan.
U.S. emergency relief teams continued to arrive in Peshawar to help, the U.S. State Department said. The U.S. Agency for International Development has committed $55 million in aid to international organizations and nongovernmental organizations, it said. The United States has provided more than 435,000 meals.
This assistance is in addition to U.S. military efforts, which include rescue airlifts, food supplies and other deliveries provided by the Pentagon.
Of the total, $20 million will be used to expand humanitarian operations farther south as the flood zone expands, Mark Ward, acting director of the Office of Foreign Disaster Assistance, told reporters Tuesday.
CNN's Reza Sayah and Samson Desta and journalist Nasir Habib contributed to this report.