President Obama announced on Tuesday a significant increase in U.S. troop levels in Afghanistan, eliciting support in Congress but also hard questions about the president’s strategy for using the troops.

In a statement, the president said he would reinforce U.S. forces in Afghanistan with two brigade-sized forces — a Marine Corps unit this spring and an Army one this summer, plus supporting troops. The additional forces would number more than 12,000 and would supplement the current force of approximately 38,000, a senior Pentagon official said. The president might add further to the buildup in subsequent months, perhaps ultimately nearly doubling the current force, officials have said.

In unveiling the long-anticipated start of a shift in forces from Iraq to Afghanistan, Obama acknowledged the need for a change in strategy in Afghanistan, which his aides in the National Security Council, Joint Chiefs of Staff and Central Command have been developing.

Saying the situation in Afghanistan and Pakistan “demands urgent attention and swift action,” Obama said the strategy is under construction. The troop increase could not wait for the strategy, he suggested.

“This increase is necessary to stabilize a deteriorating situation in Afghanistan, which has not received the strategic attention, direction and resources it urgently requires,” the president said. “That is why I ordered a review of our policy upon taking office, so we have a comprehensive strategy and the necessary resources to meet clear and achievable objectives in Afghanistan and the region.”

But the announced augmentation of troop levels will add to the pressure on Obama to articulate answers to questions about the policy those forces will be implementing.

The questions range from how much emphasis to place on securing populations and fostering development and democracy, versus limiting the focus to a mostly lethal exercise in attacking al Qaeda and the Taliban. Also of concern is the role of Pakistan and other regional powers and the extent to which tribal leaders, versus the central government, become a linchpin of security.

Complicating the picture further is the fact that the supplies needed for a growing allied force are increasingly imperiled by attacks on convoys in Pakistan.

Senate Majority Leader Harry Reid , D-Nev., anticipated the questions about the strategy in his statement.

“I support President Obama’s approval of a request from the ground commanders for more troops,” Reid said. “I also strongly support the comprehensive strategic review of our policy that is currently under way.”

Similarly, other members, including John McCain of Arizona, the ranking Republican on the Senate Armed Services Committee, said more troops alone will not lead to success in Afghanistan.

“I believe the president must spell out for the American people what he believes victory in Afghanistan will look like and articulate a coherent strategy for achieving it,” McCain said in a statement. “Today, notwithstanding the administration’s ongoing policy reviews, there still exists no integrated civil-military plan for this war — more than seven years after we began military operations. Such a strategy should spell out the way forward, including the additional resource requirements for its execution.”

John M. McHugh of New York, the ranking Republican on House Armed Services, echoes that theme.