Senate Armed Services Chairman Carl Levin is telling President Obama he should stop insisting Afghan President Hamid Karzai sign a security agreement by the end of the year.

The White House has threatened to pull all U.S. forces out of Afghanistan by the end of next year if Karzai doesn't sign the agreement by end of this year. Separately, Secretary of State John Kerry has suggested the U.S. could bypass Karzai and have his defense secretary sign the agreement instead. Levin says threats are a mistake.

"Public demands that President Karzai sign the agreement by the end of the year, or the recent suggestion that we could settle for less than an Afghan president approving the agreement, contribute to President Karzai's mistaken belief that the United States needs Afghanistan more than Afghanistan needs the United States," Levin wrote in a letter to President Obama.

Karzai has insisted the agreement should be signed the next Afghan president - whoever wins the presidential election scheduled for next April. Karzai is not running for reelection. The White House has strongly criticized Karzai for taking that stance, but Levin says it may be better to have next Afghan president sign the letter.

"The next Afghan president, whoever he is, is also likely to be more reliable than President Karzai, and there would be greater confidence in his sticking with an agreement he has signed," Levin writes.

The White House has said that waiting until April is not possible because the military will need time to plan for either withdrawing or keeping a force in Afghanistan after next year. But Levin disagrees, saying planning can happen regardless of whether the agreement is signed before the end of the year.