"At least 15,000 by the end of this year. That's what I think would be considered significant," Senator Carl Levin, a Democrat, told reporters outside the Senate.
Obama said on Monday he will soon announce his decision on how many U.S. troops to withdraw from Afghanistan beginning in July to start a gradual pullout from that country.
The White House says Obama expects to receive a recommendation from his military commanders on the withdrawal, the start of a transition to Afghan security control scheduled to conclude in 2014.
Levin's suggestion appeared to put him at odds with the ranking Republican on his committee, Senator John McCain. McCain told a newspaper this week that he hopes Obama will withdraw no more than 3,000 troops starting in July.
If Obama decides on a faster pullout as a consequence of the killing of al Qaeda leader Osama bin Laden on May 2, some experts believe he could withdraw 5,000, a full brigade combat team.
"I think the public wants the president to make a significant reduction in troops in July as he said he would do a year and a half ago and reiterated about a month ago," Levin said. "It's a critically important issue -- both for our people and for the Afghan people, to make sure that they understand the importance of continuing to take added responsibility for their own security."
The United States has 100,000 troops in Afghanistan, a deployment that costs more than $110 billion a year.
Levin said he doubted Obama would make his announcement on troops "before a couple of weeks." "I hope it's significant, believe it will be, and it should include both combat and support troops," he said.
Levin said the Afghan army's shortfall is on the so-called support side, in logistics and intelligence. (Reuters)