The U.S. ambassador to NATO says a U.S.-Afghan security agreement that would help unblock billions of dollars in aid for Afghanistan after 2014.

The deal hangs in balance because Afghanistan's President Hamid Karzai has refused to sign it. The future of NATO's nearly 12-year-old mission to Afghanistan is also tied to the outcome of this standoff. The issue will likely be at the center of discussions when the alliance's foreign ministers meet on Tuesday and Wednesday.

U.S. Ambassador Douglas Lute told reporters Monday the deal is an important "first link in the chain" that could ultimately bring more than $8 billion for Afghan security forces and development assistance after NATO ends its combat mission at the end of 2014.