Richard Holbrooke, the U.S. special envoy to Pakistan, right, visits the shrine of a Sufi saint with Pakistan Foreign Minister Shah Mahmood Qureshi in Multan on Thursday. (Khalid Tanveer/Associated Press)

The world will only be able to fund about 25 per cent of the tens of billions needed to rebuild Pakistan after the floods, and its government will have to make up the shortfall, the U.S. envoy to the country warned Thursday.

Richard Holbrooke said the U.S. would not condition its assistance to the country, but warned that Congress might not be generous if it felt that Pakistan was not taxing its own citizens enough.

Pakistan's rich have traditionally not paid much tax on their income or their property — either because they evade them or are exempt — and the country's collection rates are among the lowest in the world.

Critics have pointed to this shortage of revenue in recent weeks as Pakistani leaders have sought international aid. The country's economy is surviving on international assistance, and the floods are expected to badly slow economic growth further.

"I don't want to withhold money they need, but I think we have to be clear that the Congress is going to be reluctant to give money if the money is filling in a gap because people are not paying taxes," said Holbrooke during a visit to Karachi.

Monsoon rains triggered massive floods six weeks ago that spread across the country and are still continuing in parts of the south.

Some eight million people have been made homeless in what Pakistani and UN officials have said is one of the largest humanitarian disasters in living memory.