Severe monsoon rains have devastated northwestern Pakistan, stranding residents and washing away bridges. Drag the slider up in this interactive photograph to see the Indus River as it appeared on July 18. Pull the slider down to see the flooded region on Aug. 8. (NASA)

Extreme flooding and heavy monsoon rains in parts of Pakistan are limiting the ability of Pakistani officials and international relief organizations to deliver emergency aid, a UN agency says.

The UN has called for an increase in aid for those affected by the floods, saying the number of people either directly or indirectly affected by flooding over the past two weeks has reached 13.8 million.

The death toll stands at more than 1,500, but that number is expected to keep rising as floodwaters spread through communities in Punjab and Sindh province, officials said.

The UN's refugee agency said Tuesday that relief efforts are getting "bogged down" in some communities, as flooding washes out bridges, roads and local infrastructure.

"We are smack in the middle of a catastrophe, people in need are everywhere, some routes are blocked, and even when we deliver tents some people may lack dry land where they can be erected," said Mengesha Kebede, the refugee agency's representative in Pakistan.

United Nations Secretary General Ban Ki-moon has made an appeal for aid, saying that the relief efforts will be a "major and protracted task."

With forecasters predicting more rain, John Holmes, the UN's Under-Secretary-General for Humanitarian Affairs, said the humanitarian relief needs to be scaled up "as fast as we can."

Nadeem Sarwar, a freelance reporter in Pakistan, told CBC News that Pakistani authorities have been struggling to meet the needs of people affected by the catastrophic floods.

Sarwar said many flood victims are facing extreme shortages of clean water and medical supplies, adding that there are reports from the northwest of outbreaks of cholera and other waterborne diseases.

Flood of the century

"I think this is the flood of the century in Pakistan, if not the millennia," said Mian Gul Akbar Zeb, the high commissioner of Pakistan to Canada.

"We are a monsoon country, where we have rains for two months and are used to flooding, but the magnitude of this flood has overwhelmed the entire populations," he told CBC News.

The high commissioner said the main priorities right now are delivering clean water, food and shelter to affected communities, but he said bad weather and devastated infrastructure have made it "difficult to reach every single individual."

The hardest hit region has been the northwest, which also at the centre of Pakistan's fight against al-Qaeda and the Taliban. Sindh and Punjab provinces have also been affected.

Rescue work has been hampered by ongoing monsoon rains, which have washed away roads and bridges and made it impossible for helicopters to fly at certain times.

Pakistani president returns amid criticism

Meanwhile, Pakistani President Asif Ali Zardari returned to his flood-ravaged country Tuesday, facing a barrage of criticism for visiting Europe as his country faced its worst natural disaster.

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Families wade through water while fleeing the flooded village of Karam Pur, 70 kilometres from Sukkur in Pakistan's Sindh province on Tuesday. ((Akhtar Soomro/Reuters))

The widespread crisis has overwhelmed the government and frustrated citizens who have complained about slow or non-existent aid efforts.

Amid the relentless rains, Zardari — an unpopular figure to begin with — left for a visit to France and Britain. His aides said he had to complete the trip for diplomatic reasons, especially in the U.K., where Prime Minister David Cameron had recently accused Pakistan of exporting terror.

Akbar Zeb told CBC News that the government has mobilized the entire armed services, the paramilitary service, police and government departments to assist with flood relief.

"There's something like a million people already employed in trying to reach out to people who have been affected," he said.

"But the sheer scale of it and the time that the flood came … it makes it difficult for the government to reach everyone."

Zardari returned first to the southern city of Karachi and was expected back in the capital, Islamabad, on Wednesday. He is set to meet with the chief ministers of the provinces to map out a rehabilitation program, said Fauzia Wahab, spokeswoman for the ruling Pakistan Peoples Party.

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With files from The Associated Press