Pakistan has issued a red alert as floods that have devastated northern areas sweep south into Sindh province.
Authorities have evacuated more than half a million people living near the Indus river as hundreds of villages have been inundated by floodwaters.
The worst floods in the region for 80 years have killed at least 1,600 people and affected about 12 million others.
Meanwhile, President Asif Ali Zardari has rejected criticism that he should return from an extended foreign trip.
In a BBC interview, Mr Zardari said the cabinet was directing relief efforts, and he was being kept up to date about the situation.
"I'm the one who's given all the powers from the presidency to the parliament. The parliament is in session - the Senate is in session. It's the prime minister's responsibility, and he's fulfilling his responsibility."
The president said he had secured promises of assistance from the countries he had visited - the UAE, France and the UK.
Pakistan's meteorological office has warned that at least two more days of rain are expected in Sindh, where a red alert is in place because of the "imminent" and "extreme" flood threat.
Further downpours are also forecast in the badly-hit north-western province of Khyber-Pakhtunkhwa.
The continued bad weather has grounded all the helicopters working to deliver aid and rescue stranded survivors, according to Amal Masud of the National Disaster Management Authority.
Prime Minister Gilani called the flooding the worst in Pakistan's 63-year history and appealed for help.
"I would ask the international community to support and help Pakistan alleviate the sufferings of its flood-affected people," he said.
"The next two days are very critical. Our top priority is to rescue people, to save their lives. But we will also provide them all facilities, and we will work for their rehabilitation."
The authorities have so far evacuated hundreds of thousands of people from low-lying areas along the Indus river, much of which is already submerged.
Engineers have also warned that the huge Tarbela and Mangla dams are dangerously close to their maximum levels.
Earlier, a dam in northern Sindh's Kashmore district was breached, inundating large parts of the surrounding area with floodwater and forcing thousands of residents to take shelter on rooftops or in trees. One man told the BBC said his entire village had been destroyed and all its livestock washed away.
The BBC's Shoaib Hasan in Karachi say the authorities are now trying to prevent a breach in another dam downstream, close to the cities of Kandhkot and Kashmore, where nearly half a million people live. A breach would also threaten the Qadirpur gas field, one of Pakistan's largest.
But despite the threat of further deluges, many people have refused to leave their land, crops and homes.
"We are compelling them to evacuate because there is massive danger to their lives," Irrigation Minister Jam Saifullah Dharejo said.
Officials say 650,000 homes have so far been destroyed, 1.4m acres (557,000 hectares) of agricultural land has been flooded and 10,000 cows have died.
The UN earlier said the disaster was "on a par" with the 2005 Kashmir earthquake, which killed about 73,000 people, in terms of the damage to infrastructure and the numbers of people needing assistance.
Many of those affected by the flooding have been critical of the government's response.
"Floods killed our people, they have ruined our homes and even washed away the graves of our loved ones," Mai Sahat told the Associated Press near Sukkur, Sindh. "Yet we are here without help from the government."
Correspondents say that with victims bitterly accusing the authorities of failing to come to their aid, the disaster has piled yet more pressure on an administration struggling to contain the Taliban and an economic crisis.
Meanwhile in Indian-administered Kashmir, rescue workers are recovering more bodies after the mountainous region of Ladakh was hit by flash floods on Friday. One-hundred-and twenty people are known to have died and hundreds more are missing. Many buildings were damaged.