United States President Barack Obama has been re-elected to a second term, defeating Republican challenger Mitt Romney.

With results in from most states, America's first black president has secured the 270 votes in the electoral college needed to win the race.

Mr Obama prevailed despite lingering dissatisfaction with the economy and a well-funded challenge by Mr Romney.

Many states are still reporting results, and Mr Obama's margin of victory was not yet certain.

With swing states Virginia, Florida and Colorado still too close to call, Mr Obama has won 281 electoral votes to Mr Romney's 203.

Under the US constitution, each state is given a number of electoral votes in rough proportion to its population. The candidate who wins 270 electoral votes - by prevailing in the mostly winner-take-all state contests - becomes president.

The popular vote, which is symbolically and politically important but not decisive in the race, remains too close to call.

On Tuesday, the president held the White House by assembling solid Democratic states and a number of important swing states such as Iowa, Pennsylvania, Michigan, Minnesota and Wisconsin. His narrow victory in Ohio, a critical Mid-Western swing state, sealed the victory.

Mr Romney won North Carolina and Indiana, two states Mr Obama won in 2008, as well as the solid Republican states.

But he was unable to win in Ohio or other states needed to breach the 270 threshold.