A day after international donors pledged more than $5 billion to Pakistan, U.S. envoy Richard Holbrooke said Saturday that the aid exceeded expectations but far more is needed to strengthen the country's economy and fight terrorism.

Holbrooke, President Barack Obama's special representative for Afghanistan and Pakistan, said $5 billion is "not enough" for a big country that faces an unstable economy, complex politics, an ongoing battle against groups like the Taliban and Al Qaeda, and has an expanding nuclear arsenal.

"That's a very full agenda for any one country, and yet that's the situation Pakistan faces today," Holbrooke told reporters in Tokyo before heading back to Washington.

On Friday, representatives from 31 countries and 18 international organizations met in Tokyo for a World Bank-supported donors' conference for Pakistan.

The U.S. and Japan pledged $1 billion each, while Saudi Arabia added $700 million and the EU $640 million. The total amount pledged was $5.28 billion, according to Pakistan's foreign minister.

The donors said their contributions would focus on improving the economic climate in Pakistan through infrastructure and other projects, and stressed that stability in Pakistan was key to averting the growth of terrorism throughout the region.

"It's a sign of the fact that there's a growing awareness in the world that Pakistan is at the very heart of the threat to stability in the world," Holbrooke said, adding that the country is critical to achieving success in neighboring Afghanistan.

The total fell short of Pakistani President Asif Ali Zardari's hope of as much as $6 billion, though the conference's Japanese hosts had said they expected a figure closer to $4 billion.

Holbrooke declined to specify how much more money might be needed, and instead praised Japan for its leadership in bringing together an unlikely group of donors.

"There's no other country in the world that could have produced an outcome such as the one produced yesterday — $5 billion, coalitions of countries that barely talk to each other in other forums," said Holbrooke, a former U.S. ambassador to the United Nations and former assistant secretary of state for East Asian and Pacific affairs.

Both Japan and the U.S. will make their contributions over the next two years, and neither represented a dramatic change in their current pattern of donations. Saudi Arabia's pledge will also be disbursed over the next two years, and the EU's over the next four years.

The U.S. said in a statement it would contribute $1 billion as a "down payment" on aid it has already announced.

Holbrooke described his current job as the most challenging assignment of his career and said that Obama "inherited a very, very difficult situation" that developed in part due to past mistakes.

The administration is "re-examining every aspect" of U.S. policy toward the region, he said.