BOGOTA (Reuters) - Peace talks between the Colombian government and Marxist insurgent group the National Liberation Army (ELN) will begin only after a simultaneous release of a politician held by the guerrillas and two imprisoned rebels, an ELN leader said.

Colombia’s second-largest leftist rebel group and the government of President Juan Manuel Santos have agreed to begin peace negotiations in Ecuador, part of Santos’ efforts to end a 52-year conflict that has killed more than 220,000 and displaced millions.

But despite more than two years of preliminary talks, formal negotiations were this month postponed until January. The government has said the group must release ex-lawmaker Odin Sanchez, held in the jungle province of Choco, and stop attacks against oil infrastructure.

“We agreed that all the liberations which have yet to take place will be done simultaneously and ahead of the start (of talks),” Israel Ramirez, head of the ELN’s negotiating team and better known by his nom de guerre Pablo Beltran, told Reuters by phone.

“The same day our two are pardoned, Mr. Sanchez will be free,” said Ramirez.

Santos won the Nobel Peace Prize this year for his efforts to end the civil war.

Ramirez, a member of the ELN’s central command, did not name the two rebels he said should be released. The government said this week they will only be pardoned if they are legally eligible.

“We agreed they will happen simultaneously to eliminate mutual distrust,” Ramirez said. He ruled out a unilateral ceasefire until an agreement is reached with the government at talks.

Ramirez also denied any division regarding the peace talks within the 2,000-strong ELN, which is considered a terrorist group by the United States and the European Union.

Larger rebel group the Revolutionary Armed Forces of Colombia, or FARC, late last month signed a revised peace deal with the government, after an initial agreement was rejected in a public referendum. The fraught negotiations between the two sides lasted four years.

As with the FARC talks, Colombians should not expect a short process with the ELN, Ramirez said.

“To do it well you have to do it with calm and a lot of caution. I don’t mean they will be eternal conversations, but you can’t end a half-century conflict in a couple of weeks,” he said.

The group will for now not halt extortion against mining and oil projects, Ramirez said.

Under a peace deal, the rebels would seek power as a political party in alliance with other leftist groups, he added.