The blunt statement from Anders Fogh Rasmussen Wednesday at the start of a NATO defense ministers meeting ratchets up pressure on Afghan President Hamid Karzai to strike a deal.
On Tuesday, President Obama threatened to withdraw all U.S. troops from Afghanistan by the end of this year if the pact isn't signed.
Without that agreement, Rasmussen says, forces from other NATO countries and partners cannot stay beyond 2014 either. There are roughly 19,000 non-U.S. forces now in Afghanistan.
"Let me stress, this is not our preferred option," Rasmussen said. "But these are the facts."