"We need to draw the attention that the emergency in Pakistan is not over," said Mengesha Kebede, the UNHCR's representative in the country.
"Making sure 20 million people are rehabilitated I think is an international obligation: we are looking at a geopolitical situation where the stability of Pakistan we feel is in everybody's interest," he told journalists in Geneva.
Donors have so far funded just one-third of the 2.0 billion dollar UN aid appeal, while the UNHCR's shelter needs are only half funded, a situation Kebede dismissed as "unacceptable."
In hard-hit Sindh and Baluchistan provinces, a total of about 2.7 million people are still displaced.
Kebede underlined that about 600,000 displaced had sought shelter in schools in Baluchistan, where local authorities wanted to reopen them to start schooling again.
However, many of the displaced could not return to damaged homes and supplies to set up huge tented camps were short.
"That kind of challenge continues on a daily basis in every province," Kebede added.
UNHCR is also caring for the world's largest refugee population in Pakistan, some 1.7 million Afghan refugees, many of whom were in flood hit areas.
Another 1.2 million Pakistanis are still displaced by conflict in the northern Khyber Phatunkhwa province.
"That problem has become secondary because of the magnitude of the floods. Now that the floods have started receding it will definitely start surfacing again because the conflift continues," said Kebede.
Pakistan's fraught political climate has been scarred by violence in recent years, while the country's anarchic northwestern tribal belt remains a reputed hideout for foreign and homegrown militants linked to the Taliban and Al-Qaeda.
Last month an unemployed father of four who lost his home in the floods died after setting fire to himself outside the prime minister's home.