US Republican presidential nominee Mitt Romney (L) answers a question as US President Barack Obama listens during the final US presidential debate in Boca Raton, Florida, Oct 22, 2012. [Photo/Agencies]

BOCA RATON, the United States - US President Barack Obama and his Republican rival Mitt Romney were starting on Monday night to duel on foreign policy issues in their third and final face-to-face encounter.

The debate came 15 days before voters choose a new White House occupant.

The face-off in Lynn University in Boca Raton of Florida could turn out consequential in a very tight race as latest polls show the duo neck-and-neck among likely voters.

Moderator Bob Schieffer from CBS News has selected five topics for the 90-minute event, namely America's role in the world, the war in Afghanistan, Iran's nuclear issue, the changing Middle East and the new face of terrorism, and the rise of China.

While foreign policy has been a strength of Obama throughout the campaigns, the Democratic incumbent has found his advantage narrowing to a mere four percentage points, a contraction of 11 points from last month, following his lackluster performance in the first presidential debate on Oct 3, and his rebound in the next encounter on Oct 16 has failed to regain the lead.