First visit by a US president to former pariah state; Obama faces criticism from human rights groups for going too soon but is expected to announce resumption of US aid.
President Barack Obama repeatedly attacked Republican challenger Mitt Romney on foreign policy on Monday in their third and final debate in an effort to blunt his opponent's surge in the polls with two weeks left until Election Day.
The two leaders appeared to share little common ground on the fate of Syrian President Bashar al-Assad, and their mostly chilly personal encounter suggested a less-than-promising start.
Obama's efforts to spur global cooperation to prevent nuclear terrorism have been overshadowed by the more urgent challenges of thwarting Iran's and North Korea's nuclear development.
"While no decision has been made, we have serious concerns about the deteriorating security situation in Damascus," the State Department said in a written statement.
President Barack Obama served notice on Sunday that the United States was fed up with China's trade and currency practices as he turned up the heat on America''s biggest economic rival.