In the document, the guerrillas propose to create, "in mutual agreement" with the incoming government, a "climate conducive to a bilateral ceasefire agreement."

The agreement would make it possible to find "a political solution to the violence," according to the rebels.

The letter, dated July 31, was published on Wednesday by several media outlets along with a video showing 18 men and women dressed in camouflage and carrying rifles.

On July 15, outgoing President Ivan Duque announced that during a military operation, Ivan Mordisco, the leader of the organization known as the Southeast Bloc, had died.

With some 2,000 members, according to the Indepaz think tank, the Southeast Bloc is the largest of the dissident groups operating in Colombia and has a presence mainly in the Amazon jungle and along the Venezuelan border.

In their letter, however, the rebels said one of the "plenipotentiaries" that would negotiate with Petro's government would be "Ivan Lozada," an alias of "Ivan Mordisco." They did not clarify if this was the same person who had been announced as dead.

Petro, a former guerrilla and senator, will take office Sunday and has expressed an interest in reaching a "total peace" with all armed groups operating in Colombia.

The National Liberation Army (ELN), the last recognized guerrilla group in the country after the disarmament of the FARC, expressed its intention to negotiate with Petro shortly after his election as Colombia's first leftist president.

The new president also proposed the "peaceful dismantling" of other organizations such as the Clan del Golfo, the country's largest drug cartel, which two weeks ago announced its willingness to coordinate a ceasefire to negotiate its disarmament.