ABIDJAN, July 11 (Reuters) - Ivory Coast has asked the United Nations to investigate an assassination attempt against Prime Minister Guillaume Soro, the former French colony's government said on Wednesday.

Unidentified attackers fired rockets at the Fokker 100 aircraft carrying Soro moments after it landed in the northern rebel stronghold of Bouake last month, threatening to delay already slow progress on a peace plan in the world's top cocoa grower.

Soro, leader of the New Forces rebels, was made prime minister in April under a deal to reunite the West African country, split in two since the rebels seized its northern half during a brief 2002-03 civil war.

"(The U.N. investigation) will begin immediately, in the coming days. The president of the republic has given instructions for that to happen," government spokesman Amadou Kone told reporters after a cabinet meeting.

Many observers suspect disgruntled rebels of the attack on their leader in a bid to thwart progress on a deal which would call on them to disarm and forfeit power, status and wealth.

Others suspect it could have been carried out by sympathisers of his former arch-enemy, President Laurent Gbagbo.

Four of Soro's aides were killed and several more injured in the attack. Soro has denied his military chiefs were involved and both he and Gbagbo have said the incident has stiffened their resolve to press on with the peace process.

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