* Fighters attacking Ivory Coast from Liberia
* Supporters of former Ivorian president behind violence, U.N. says
By Loucoumane Coulibaly
ABIDJAN, Oct 18 (Reuters) - West African neighbours Ivory Coast and Liberia will launch a joint military operation by the end of the year along their volatile and porous border, the two nations' presidents announced on Thursday.
Western Ivory Coast has been the target of deadly raids blamed on supporters of former president Laurent Gbagbo, who was ousted in a civil war last year after he rejected the election victory of rival Alassane Ouattara.
In a report published on Wednesday, U.N. investigators said pro-Gbagbo exiles were using Liberian territory as a "recruitment platform and rear base" for operations aiming to topple Ouattara, who is now president.
"I want to reassure you that we will continue our surveillance, our monitoring, our intelligence gathering and will take whatever action is necessary to ensure that peace and tranquillity prevails," Liberian President Ellen Johnson Sirleaf said on Thursday following a meeting with Ouattara in Ivory Coast's commercial capital Abidjan.
Johnson Sirleaf and Ouattara said in a statement that their two governments had decided "to organise a joint military and security manoeuvre before the end of 2012".
Interior, defence and foreign affairs ministers from the two countries have met three times in recent months to calm tensions over the border violence.
While Gbagbo is currently in The Hague charged with crimes against humanity by the International Criminal Court, many of his top political and military allies are living in exile in neighbouring West African nations.
In their report, the U.N. experts, who are charged with monitoring an arms embargo imposed on Ivory Coast, said leading members of the Gbagbo regime had established a "strategic command" in Ghana.
The report also said that the group had organised the recruitment of Liberian mercenaries to fight alongside Ivorian militias who crossed into Liberia at the end of last year's conflict.
Liberia announced on Thursday that it had arrested Bobby Sarpee Julu, one of the top military commanders of the cross-border operations against Ivory Coast cited in the U.N. experts' report.
"Sarpee Julu, a Liberian but fighting for the Ivorian rebels was arrested on Monday in (the town of) Ganta ... Julu is one of the ringleaders for the Ivorian rebel group," Information Minister Lewis Brown told Reuters.
Liberia transferred 41 suspected Ivorian fighters to Ivory Coast earlier this year. It is also currently holding several Liberian mercenaries believed to have killed seven United Nations peacekeepers near the Ivorian town of Tai in June.
Of around 200,000 Ivorian refugees who fled to Liberia during and after the war, around 60,000 remain there, many in camps not far from the border. (Additional reporting by Alphonso Toweh in Monrovia; Writing by Joe Bavier; Editing by Richard Valdmanis and Myra MacDonald)