HAVANA (Reuters) - Colombian President Juan Manuel Santos’s effort to negotiate a ceasefire with the country’s last rebel group before stepping aside next week has failed, the two parties said on Wednesday.

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The government, in a joint statement with the National Liberation Army (ELN), said advances were made in a sixth round of talks in Havana toward a ceasefire but “others needed to make more effective its implementation and verification were left pending.”

The statement said progress had been made since the talks began in 2017 and dialogue was the best option to settle their differences.

Santos, who signed an historic peace accord with the Revolutionary Armed Forces of Colombia (FARC) in 2016, sent Foreign Minister Maria Angela Holguin to Cuba over the weekend in a last-ditch attempt to reach a temporary halt to hostilities before right-wing President-elect Ivan Duque takes office on Aug. 7.

Peace talks with the ELN are being held in Havana, the venue where the agreement with the much larger FARC was brokered.

“We are not completely satisfied because we did not achieve a ceasefire,” lead government negotiator Gustavo Bell said, closing the latest round of talks.

Rebel negotiator Pablo Beltran said he hoped the progress made would lead to continued negotiations with the Duque administration.

The ELN, through its twitter account, called on the “international community, the church (and) political and social movements to insist the government of president Duque continue the peace process.”

Duque has promised to make changes to the peace accord to jail former FARC commanders who committed crimes.

Santos has been criticized by many Colombians who say he sold out to the rebels in return for a Nobel Peace Prize.

But Santos, in a recent interview with Reuters, said it will be almost impossible to change the accord, which has been endorsed by the international community and broadly supported by the constitutional court.

“Peace can’t be changed for many reasons and for ethical and moral reasons - nobody wants to go back,” he said.

Colombia’s conflict between the government, rebel groups, paramilitaries and crime gangs has lasted more than 50 years, leaving at least 220,000 people dead and millions displaced.

Colombia has been at war with the 1,500-member ELN, which was founded by radical Catholic priests, since 1964.