By Michelle Nichols

NEW YORK, Feb 25 (Reuters) - The United States must focus on securing and rebuilding Afghanistan because if it fails then neighboring Pakistan could follow, U.S. Sen. Joe Biden said on Monday after returning from a tour of both countries.

Biden, chairman of the Senate Foreign Relations Committee, said more troops are needed in Afghanistan and called for greater focus on basics like roads and power plus giving the military cash for quick projects like digging wells.

He also urged a rebuilding plan similar to the Marshall Plan under which the United States aided Europe's shattered economies after World War Two.

"Afghanistan's fate and Pakistan's future are joined and America's security is tied to both," Biden told the Council on Foreign Relations. "If Afghanistan fails, Pakistan could follow, because extremists will set their sights on the bigger prize to the east."

Frustration is rising among many ordinary people in Afghanistan over the perceived lack of development and security Western leaders promised before the Taliban were driven from power in 2001.

U.S.-led forces overthrew the Taliban's government after it refused to hand over al Qaeda chief Osama bin Laden, whom Washington says is the architect of the Sept. 11 attacks on the United States.

The militants have made a comeback in the past two years and violence is at its worst since the Taliban's fall.

More than 50,000 foreign troops led by NATO and the U.S. military are in Afghanistan battling Islamist militants who have found refuge in lawless border areas with Pakistan.

"We have spent on Afghanistan's reconstruction in six years what we spend every three weeks on military operations in Iraq," Biden, a Democrat who withdrew from the presidential race last month, said.

Biden, Democrat Sen. John Kerry and Republican Sen. Chuck Hagel visited Pakistan, India, Turkey and Afghanistan.

Biden said that if the United States makes Afghanistan its priority, then so will its allies.

"It seems time for NATO to realize that they must get fully in the fight. If Afghanistan falls, I am not sure how far behind NATO will be," he said. "If America does more, so will our allies."

Biden said Pakistan's cooperation in the fight against extremism was also critical to the success of Afghanistan but had so far been "sporadic at best," adding that Washington had to move from a policy focused on a personality -- Pakistan's President Pervez Musharraf -- to one focused on the country.

He said the United States needed to triple its nonmilitary assistance to Pakistan and sustain it for a decade focusing on schools, roads and clinics, give the government a "democracy dividend" above this to jump-start progress, help Islamabad develop the country's northwest provinces and demand transparency and accountability in the military aid provided.

(Editing by Cynthia Osterman)

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