The number of people affected by catastrophic floods in Pakistan may outnumber those suffering from the recent major natural disasters -- the 2004 Indian Ocean tsunami, the 2005 South Asia earthquake and the 2010 Haiti earthquake, the United Nations said on Monday.

"The number of people affected by Pakistan's floods is now estimated to be 13.8 million, according to the government of Pakistan," said a press release from the UN Office for the Coordination of Humanitarian Affairs (OCHA).

"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 (five million), the 2005 South Asia earthquake (three million), or the 2010 Haiti earthquake (three million)," the press release said. "The estimate of homes destroyed or seriously damaged -- 290,000 -- is almost the same as those destroyed in Haiti."

Earlier on Monday, UN Secretary-general Ban Ki-moon voiced his extreme concern over the massive floods that have killed hundreds of people in Pakistan, and urged donors to contribute generously to the humanitarian response, saying the effects of the disaster rivaled the impact of the earthquake that struck the South Asian country in 2005.

The earthquake is estimated to have claimed the lives of 80,000 people and caused widespread destruction in areas around Pakistan's border with India.

"The scale of this disaster [floods] rivals that of the earthquake in October 2005, but this time the geographic range is much greater," the secretary-general said during his monthly press conference at the UN Headquarters in New York.

"Let me stress now that we must also give thought to medium and longer-term assistance. This will be a major and protracted task," Ban said. "I appeal for donors to generously support Pakistan at this difficult time."