(Updates with president's comments paragraphs 1, 8-10)

By Salim Bamba

BOUAKE, Ivory Coast, June 30 (Reuters) - A rocket attack on Ivory Coast's prime minister has shaken the West African state's fledgling peace process but his rebel movement and the president vowed on Saturday to carry on reuniting the war-divided state.

A rocket hit the plane of premier Guillaume Soro, who also leads New Forces rebels controlling the north of the world's top cocoa grower after a 2002-03 civil war, as it touched down in the main rebel city, Bouake, on Friday, killing four aides.

"Those planting bombs can carry on if they want, but they should know these actions are destined to fail," New Forces spokesman Sidiki Konate said on Saturday at a meeting to reassure worried residents in the rebel city of Bouake.

Konate said several people had been arrested after Friday's attack, but did not say who they were.

Analysts have suggested disaffected New Forces fighters may have been responsible, but neither the rebel leadership nor President Laurent Gbagbo's administration in the south of the country have said who they think launched the attack.

Even the usually ferociously partisan press avoided speculation in Saturday's editions over who did it.

Formerly arch enemies, Soro and Gbagbo signed a peace deal in March in which they agreed to reunite the country and organise long-delayed elections, offering optimism among the war-weary population that the standoff was finally over.

"You don't assassinate peace... It is a vain attack because it can't stop the peace process," Gbagbo told reporters at his home, adding that both he and Soro were aware some who were "upset" by the peace momentum were plotting to thwart it.

"It's an illusion to think that we're going quietly back to peace without unsettling the devil," he said, praising his former enemy Soro for his courage and calm following the attack.

Gbagbo said he would delay his first trip to Bouake since the war, scheduled for July 5, until later in the month.

PRESSING ON

Hours after the attack, a ceremony marking the official return of magistrates to the north of the country as part of the peace plan, for which Soro had travelled to Bouake, went ahead as planned, though Soro did not attend in person.

Several hundred residents briefly marched in Bouake on Saturday to mark their support for Soro and the peace process.

"Since yesterday the mistrust has returned. We now have proof the enemies of peace exist and will do anything to rouse the demons of violence, the demons of war," Bouake pensioner Eric Kone said at a meeting on how the country's various ethnic communities can contribute to peace.

Ethnic rivalries between the mostly Christian cocoa-producing southern part of the former French colony and the predominantly Muslim north has fuelled years of political violence in the country that culminated in civil war in 2002.

Last week the International Crisis Group think-tank warned the peace process had not yet addressed key causes of the war.

"We're tired of fighting and killing each other. It's time that those who are behind yesterday's terrible events understand that war is no longer possible and that we've committed to an irreversible process," said Corporal Adama Sylla, a New Forces soldier standing on duty at Bouake's football stadium.

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