* Diplomats say Western powers recognise New Forces as army
* Ouattara can be held accountable for any crimes by gunmen
ABIDJAN, March 17 (Reuters) - Ivory Coast's Alassane Ouattara broke three months of silence on the gunmen fighting to defend his claim to the presidency on Thursday, saying he officially recognised the former rebels as the legitimate army.
A dispute between Ouattara, whose claim has U.N. backing, and incumbent Laurent Gbagbo over a November election has plunged Ivory Coast into conflict, escalating in the past three weeks into heavy fighting in the main commercial city Abidjan.
It also triggered a resumption of hostilities in the west of the world's top cocoa grower, across a north-south ceasefire line between the northern rebel forces and a Gbagbo-controlled south that has been in place since a 2002-3 civil war.
Security in the main commercial city has been rapidly deteriorating since gunmen claiming allegiance to Ouattara, including some army defectors, took over a northern suburb last month, and this week used it as a base to push south and west.
"I have just made a decree for the creation of the Republican Forces of Ivory Coast (FRCI)," Ouattara said in a statement. "This new army is composed of the national army and the (former rebel) New Forces."
It was the first time Ouattara acknowledged command over the former rebels and Abidjan gunmen fighting to oust Gbagbo, who has refused to step down with support of most of his military, despite losing a U.N.-certified election.
Ouattara had formerly distanced himself from them.
"Their mission is principally to assure the security of people and their belongings," he said. "This new army will follow a new code of conduct in the service of its citizens."
A Western diplomat told Reuters this meant the New Forces, once regarded as mere warlords, were now officially recognised by the international community as Ivory Coast's military.
"They absolutely are the legitimate force. We have been talking to them about the responsibilities that go with that. We're calling them the RFCI, not the rebels," he said.
Putting the various pro-Ouattara forces under his command means he will be responsible for any crimes they commit.
Human rights watch has documented cases this week of pro-Ouattara gunmen executing Gbagbo's troops they captured, and called on him to stop such abuses. (Reporting by Loucoumane Coulibaly; writing by Tim Cocks; editing by Philippa Fletcher)