BOGOTA, April 7 (Reuters) - The leadership shake-up in Colombia's National Liberation Army (ELN) after the death of rebelpriest Manuel Perez could lead to splits and a hardening of its military line, army sources and ex-rebels said Tuesday.

Perez, 54, commander-in-chief of Colombia's second largest guerrilla army, died of hepatitis in mid-February. But the news was only announced on Monday by his successor and former deputy, Nicolas Rodriguez, 48, alias "Gabino."

Rodriguez, the oldest of 17 peasant children, joined the ELN as one of its founders in 1964 and has spent all his adult life in rebel ranks.

"He's a hard-liner and definitely favors the military line," a senior member of military intelligence services told Reuters, requesting anonymity.

Rodriguez took charge of the Cuban-inspired ELN's military operations while Perez mapped out political strategy, which included authorizing rebel representatives to take part in secret talks with the government in Madrid early this year.

Those meetings led to a deal to start preliminary peace talks in Colombia later this year. But the ELN, accusing the government of "political opportunism," last week announced it had decided to scrap the planned negotiations.

Former ELN rebel Fernando Hernandez, head of an ELN political spin-off called Socialist Renovation, rejected army claims that Rodriguez was an irrevocable hard-liner and said he had learned a lot of "political flexibility" from Perez.

Hernandez said he believed Rodriguez's most immediate challenge would be to maintain the unity of the 5,000-strong guerrilla force.

"The death of Manuel Perez has left the ELN without a man with the moral authority to unify the different tendencies within the movement," he told Reuters. "That could lead to a greater fragmentation or even splits."

The ELN has scores of semi-autonomous units or fronts spread across Colombia. Urban militias in Bogota and the spread across Colombia. Urban militias in Bogota and the northwestern city of Medellin are known to be hard-line and fierce opponents of any negotiated peace deal.

But Hernandez said the most likely group to break away would be the Domingo Lain Front, which he called a "fundamentalist group with dogmatic Marxist views."

The Domingo Lain, based in the northeastern province of Arauca and estimated to have about 400 fighters, has spearheaded attacks on key oil infrastructure.

The creation of splinter groups could spark an upsurge in sabotage, kidnapping and attacks on oil installations but would almost certainly reduce the overall military capacity of the ELN, Hernandez said.

In a column in the respected El Espectador newspaper, political analyst Alfredo Molano said Perez's death could also lead to a shift in relations with the larger Revolutionary Armed Forces of Colombia (FARC), Latin America's oldest and largest rebel force.

Both forces tried to establish closer military coordination through a joint command structure set up in the late 1980s.

The initiative largely collapsed due to tactical and political differences, but the FARC issued a communique early this year calling for greater unification.

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