ABIDJAN, Ivory Coast - Forces loyal to the two men claiming Ivory Coast's presidency clashed in the streets of this commercial capital Thursday, leaving at least 20 dead and bolstering fears that the country, the world's top cocoa producer, is on the verge of another civil war.

Explosions and gunfire were heard throughout Abidjan - known as the "Paris of Africa" for its cosmopolitan nightlife and chic boutiques.

An errant rocket-propelled grenade struck a perimeter wall of the U.S. Embassy, but no injuries were reported and the damage was minor, State Department spokesman P.J. Crowley said in Washington.

Ivory Coast has been operating with two presidents since a disputed Nov. 28 runoff election that was meant to end divisions left by the 2002-03 civil war.

Alassane Ouattara was declared the winner by the country's electoral commission and was recognized by the United Nations, United States, France, and the African Union as having defeated incumbent Laurent Gbagbo.

The next day, however, the constitutional council overturned the results after invalidating a half-million votes from Ouattara strongholds.

The United States and other nations have given Gbagbo an ultimatum to step down and leave Ivory Coast within days or face international travel and financial sanctions, a senior Obama administration official said.

The official said that the United States, the United Nations, France, and the African Union had passed the message to Gbagbo's camp that "he has to go" and that there were signs he might agree.

The official spoke on condition of anonymity to describe the ongoing discussions.

The official would not describe the signs but noted that Gbagbo and his family had "multiple homes in multiple countries" that they would not be able to use if sanctions were imposed.

Streams of gunfire and unexplained explosions were heard for 30 to 45 minutes in the streets outside the U.N.-protected Golf Hotel, from where Ouattara has tried to govern. Gbagbo rules from the presidential palace.

Casualty tolls for the day's violence varied. Gbagbo's minister of education, Jacqueline Oble, confirmed 20 deaths in a statement read on state television, specifying that 10 of those were police killed by protesters.

Senior opposition official Amadou Coulibaly put the toll at 30 dead.

Traore Drissa, a lawyer who runs the Abidjan-based Ivorian Movement for Human Rights, said 20 people were killed. Amnesty International counted nine bodies.

U.N. Secretary-General Ban Ki-moon has warned of a new civil war in a country known for decades as a beacon of prosperity and stability in a part of Africa known for coups and war. Its cocoa plantations attract millions of workers from neighboring nations.

Ouattara draws much of his support from the mainly Muslim north, while Gbagbo's power base is in the mainly Christian south.