WASHINGTON - President Obama's choice for U.S. ambassador to Afghanistan insisted yesterday that the United States must continue its multibillion-dollar investment to achieve a "good enough" government in Kabul that would prevent the country from backsliding into a sanctuary for terrorists.

Ryan Crocker was challenged repeatedly by skeptical senators who questioned a costly war now in its 10th year and nation-building that a fresh congressional report found has had limited success despite nearly $19 billion in foreign aid over a decade. That's more than the United States has spent in any other country, including Iraq.

"Our current commitment, in troops and dollars, is neither proportional to our interests nor sustainable," said Sen. John Kerry, D-Mass., chairman of the Senate Foreign Relations Committee.

Looming large as senators pressed Crocker to state an Afghanistan end game was Obama's upcoming decision on how many of the 100,000 American troops to withdraw from Afghanistan in July. The killing of al Qaeda leader Osama bin Laden, reservations about unreliable-ally Pakistan and significant U.S. budget constraints have forced even the more hawkish members of Congress to rethink continued U.S. involvement in Afghanistan.