* Poll dispute tipping Ivory Coast back to civil war * Fighting had been restricted to north
ABIDJAN, March 2 (Reuters) - Explosions rocked the southern Abidjan suburb of Koumassi overnight and on Wednesday, as fighting between insurgents seeking to oust Ivory Coast's Laurent Gbagbo and security forces spread to new areas of the main city.
Security in the world's top cocoa grower is deteriorating, with gun battles between rival forces most of last week and hostilities resuming across a north-south ceasefire line that had been largely quiet since a 2002-3 war ended in stalemate.
Ivory Coast has been in turmoil since a disputed November election between Gbagbo and his rival Alassane Ouattara, who won the poll according to U.N.-certified results that Gbabgo has refused to concede.
Fighting has mostly been restricted to northern pro-Ouattara suburb of Abobo, where an insurgent force calling itself the invisible commandos has seized control of most of the territory and forced out pro-Gbagbo security forces.
Gunfire has also been heard much closer to the central business district. Across the other side of town, residents of Koumassi said fierce fighting had flared up there too.
"Since last night there have been a lot of military on the streets. They said they are looking for insurgents who have infiltrated the neighbourhood," said student Eloise Kouassi. "The traffic is dead: everyone is staying in."
November's election was meant to heal divisions sown by a 2002-3 civil war that left the country divided into a rebel-run north and government-run south, but the dispute has worsened divisions and killed well over 300 people since November.
The U.N. says the number of Ivorian refugees in Liberia has reached 68,000, with another 40,000 internally displaced.
"We're hearing explosions and gunfire in Koumassi. No one is outside except the military," said Patrice Gore, 31, a mechanic who lives there. "We hit the ground every time we hear them."
On Tuesday, Gbagbo's notoriously xenophobic youth supporters rampaged through Abidjan's central business district, pillaging shops owned by foreigners.
The crisis has had a catastrophic impact on Ivory Coast's economy, with international banks shutting down, shops shuttering up and bars and restaurants empty at night. (Reporting by Ange Aboa; Writing by Tim Cocks; editing by Ralph Boulton)