The floods, triggered by heavy monsoon rain in late July, killed more than 1,750 people, forced at least 10 million people from their homes and caused up to $43 billion in damage.
"The international community is not going to be able to raise tens of billions of dollars," US Special Representative for Afghanistan and Pakistan Richard Holbrooke told a meeting of newspaper editors in Karachi.
"You have to figure out a way to raise the money," he said.
A massive cascade of waters swept through the country, washing away homes, roads, bridges, crops and livestock, sending the vital US ally in the campaign against militancy reeling in one of the worst natural disasters in recorded history.
Pakistan's economy was already fragile and the cost of rehabilitation will likely push the 2010/11 fiscal deficit to between 6 and 7 percent of gross domestic product (GPD) against an original target of 4 percent.
The floods are "going to put your government to the test", Holbrooke said.
RECONSTRUCTION WORRY
Pakistani Foreign Minister Shah Mehmood Qureshi said the government was contemplating measures to generate revenues inside the country.
"We...intend to revisit our budgetary priorities to cap non-development expenditures, to reprioritise our development allocations and to see what we can do to mobilise national resources," he told reporters in the city of Multan alongwith Holbrooke and Australian Foreign Minister Kevin Rudd.
The International Monetary Fund (IMF) on Wednesday approved as expected $451 million in emergency funding to help the country rebuild. That amount is separate from a $11 billion IMF-backed economic programme agreed in 2008.
About 10 million flood victims are in urgent need of food and shelter. Aid agencies warn that water-borne diseases and hunger could kill many more.
"I have seen many refugee camps in the last 40 years but I have not seen people along the roads in such numbers and in such desperate situations and we need to help them," Holbrooke said after visiting flood-hit areas near Multan.
The United States has taken the lead in providing emergency aid, contributing $261 million for relief and security.
