U.S. Defense Secretary Robert Gates says President Barack Obama's plan to withdraw U.S. combat troops from Iraq in 16 months is one of the options currently being studied at the Pentagon.

Secretary Gates told reporters at the Pentagon on Thursday that since the November presidential election, the military has been working on a variety of options to pull U.S. troops out of Iraq. 

"From really ever since the election, we have been looking at several options, and obviously 16 months is one of them. We are very aware of what the president has said and we have an obligation and a responsibility to provide him with a range of options that include the one that he has spoken about," he said.

The U.S. has more than 140,000 troops in Iraq. President Obama has promised to withdraw combat forces within 16 months, although he has said some troops will remain to fight terrorist cells, train Iraqi soldiers and provide security.

Some American military commanders say the United States needs to be careful about withdrawing troops from Iraq too quickly, saying progress there is fragile and could be reversed.

The defense secretary's remarks come one day after Mr. Obama met with top military leaders and told them to draft additional plans to execute a drawdown of soldiers in Iraq. A statement released after the meeting did not include a specific timetable.

U.S. Navy Admiral Michael Mullen, the Chairman of the Joint Chiefs of Staff and principle military advisor to the president, says he and Mr. Obama discussed a deliberate and rapid process to look at a responsible withdrawal of troops from Iraq.

"We have worked this very hard. We have planned an awful lot of options, but there will need to be additional meetings and engagements to work our way through the fullness of both Iraq and Afghanistan, hear from the Joint Chiefs, which is what the president has said he would do as well," he said.

President Obama supports a plan to nearly double the number of U.S. forces in Afghanistan, where violence has increased in recent months.

Late last year, NATO defense ministers gave new guidance to commanders to attack those involved in Afghanistan's illegal narcotics trade who are providing financial support to the insurgents.

Secretary Gates says he has given U.S. troops permission to do the same thing.

"We clearly have to go after the drug labs and the drug lords that provide support to the Taliban and to other insurgents. I have signed off on a change in the rules of engagement for our own forces. If we have evidence that the drug labs and drug lords are supporting the Taliban, then they are fair game," he said.

Gates also says the goals the United States has had for Afghanistan are too broad and are oriented too far into the future.

"And we need more concrete goals that can be achieved realistically within three to five years in terms of reestablishing control in certain areas, providing security for the population, going after al-Qaida, preventing the reestablishment of terrorism, better performance in terms of delivery of services to the people, some very concrete things," he said.

The United States has some 33,000 ground forces in Afghanistan. President Obama is expected to support adding up to 30,000 more troops this year as his administration shifts America's military focus from Iraq to Afghanistan.