WASHINGTON (AP) In a blunt warning to Afghan President Hamid Karzai, President Barack Obama threatened on Tuesday to withdraw all U.S. troops from Afghanistan by the end of this year if a crucial security pact isn’t signed, and he ordered the Pentagon to accelerate planning for just that scenario.

At the same time, in a rare phone call with Karzai, Obama indicated he was willing to wait out his mercurial counterpart and sign a security agreement with a new Afghan president after April elections. That would allow the U.S. to keep as many as 10,000 troops in the country.

The effort seemed aimed at marginalizing Karzai’s role in the high-stakes negotiations over the future of the lengthy American-led war.

“We will leave open the possibility of concluding a [security agreement] with Afghanistan later this year,” the White House said in a statement following the call. “However, the longer we go without a [deal], the more challenging it will be to plan and execute any U.S. mission.”

Obama’s attempt to minimize Karzai’s importance to U.S. decision-making underscores how fractured the relationship between the two leaders has become. Tuesday’s phone call was the first direct contact between Obama and Karzai since last June. The Afghan leader has deeply irritated Washington with anti-American rhetoric, as well as with his decision this month to release 65 prisoners despite objections from U.S. officials.

The White House insists it won’t keep any American troops in Afghanistan after December without a security agreement, giving the military a legal basis for staying in the country. While the White House did not publicly set a deadline for finalizing the agreement before that time, officials said the size and scope of the any U.S. mission could shrink the longer Obama waits.

Despite the troubled ties between Washington and Kabul, many of Obama’s advisers want to see American troops stay in Afghanistan after the war formally concludes in December. The Pentagon envisions keeping up to 10,000 troops in Afghanistan to focus on counterterrorism and the training of Afghan security forces, though some White House advisers would prefer keeping fewer troops, if any.

The U.S. military has also drawn up blueprints for a full withdrawal, and Tuesday’s developments appeared to push that idea closer to the forefront of Pentagon planning.

Obama’s call with Karzai coincided with key military meetings on Afghanistan. Defense Secretary Chuck Hagel will meet with his NATO counterparts in Brussels later this week. And Joint Chiefs Chairman Gen. Martin Dempsey arrived in Afghanistan on Tuesday to visit U.S. military leaders in the country and assess the security situation on the ground.

Dempsey, speaking to reporters traveling with him, said that while the U.S. remains committed to helping Afghanistan after this year, “I can’t ask the young men and women to serve in a country without the protections afforded by a bilateral security agreement.”

“We are at a point where we have to begin planning for other options,” Dempsey said.

The prospect of a full American withdrawal has led to concern among Afghanistan’s neighbors, most notably Pakistan, where officials have warned that a civil war could break out and further destabilize the region. Pakistani officials also worry that Afghan security forces will fracture and as many as one-third of the force could desert without continued U.S. assistance.

The U.S. and Afghanistan agreed to details of a security pact last year, and a council of 3,000 Afghan tribal elders known as the Loya Jirga also endorsed the agreement. But Karzai caught U.S. officials off-guard by then declaring he wanted his successor to sign the agreement.

It’s unclear whether Afghanistan’s new president will be any more likely than Karzai to do so. There is no clear front-runner among the 11 candidates running to replace the president, who is constitutionally ineligible for a third term and has not endorsed a successor.

Among those running are Abdullah Abdullah, who was the runner-up to Karzai in disputed 2009 elections; Qayyum Karzai, a businessman and the president’s older brother, and Ashraf Ghani, a former finance minister and academic. Most of the candidates are familiar to U.S. officials.

The longer the U.S. waits to make a decision on its future in Afghanistan, the more expensive and risky a full withdrawal would become. With less time to move troops and equipment, the military will have to fly assets out rather than use cheaper ground transportation.

The Pentagon’s biggest challenge will be closing large military facilities, including the Bagram and Kandahar air bases. Shutting down a massive base typically takes about 10 months, but military officials said they are prepared to do it in a much shorter although far more expensive period if necessary. Military officials said commanders would still like to have about six months.

The Pentagon is currently planning to cut the total American force in Afghanistan to as low as 20,000 by mid-summer, giving commanders the ability to pull all troops out by Dec. 31 if no agreement is reached. There are currently about 33,600 U.S. troops in Afghanistan.

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Associated Press writers Robert Burns and Deb Riechmann in Washington, Lolita C. Baldor at Bagram Air Field, Afghanistan, and Cassandra Vinograd in Kabul, Afghanistan, contributed to this report.