Democrat Barack Obama won a second term as president of the United States, defeating former Massachusetts Gov. Mitt Romney, television networks said, but the Republican hopeful's campaign said he wasn't ready to concede.

The networks called Tuesday's election for the incumbent based on Obama's leads in the swing states of Ohio and Iowa.

With 55 percent of the ballots counted in Iowa, Obama led his Republican challenger by 55 percent to 44 percent.

In Ohio, the Democrat was ahead by 50 percent to 49 percent with 73 percent of votes tabulated. In the past 50 years, no one has secured the White House without winning the Buckeye State's 18 electoral votes.

Pending final results in important states such as Florida and Virginia, Obama stood with 281 electoral votes and Romney had 203, according to a provisonal tally by media outlets.

A candidate needs 270 electoral votes to win.

Romney, however, continued to hold a razor-thin lead over Obama in the popular vote

The Romney campaign told Fox News the former governor was not prepared to concede the election until all the votes are counted in Ohio and Florida.

"This happened because of you. Thank you," Obama told his supporters on Twitter after the networks projected him as the winner.

"Four more years," he added.

The nation's first black president will again face a divided Congress when he begins his second term in January, as media projections showed the GOP retaining control of the House of Representatives and Democrats maintaining a majority in the Senate. EFE