Colombia's Marxist FARC rebels pledged on Thursday to honour its commitment to completely disarm by June 20 after a UN monitoring mission said it had received fewer weapons than expected.

"We made the political decision, we respect the agreement and we will apply it whatever happens," FARC's leader, Rodrigo Londono, also known as Timochenko, told reporters in Oslo.

He was speaking after a public meeting and discussion with Colombia's Foreign Minister Maria Holguin.

Under a historic peace agreement signed last year with the Colombian government to end a half-century of war, the FARC had to surrender their weapons to the UN mission in Colombia before the end of May. That deadline was postponed to June 20 due to logistical problems.

After first delivering 30 percent of their weapons on June 7, the guerrillas were expected to hand in another 30 percent by Wednesday, with the rest due by June 20.

A UN mission in Colombia however said it had to date only received about 40 percent, and not the 60 percent expected.

Holguin nevertheless said she was confident the FARC would deliver.

"I think the FARC will fulfil that commitment," she said, acknowledging the logistical problems posed by the disarmament process.

"These are facts that make the process of peace" credible, she said.

The Colombian conflict erupted in 1964 when the FARC and the ELN, a smaller rebel group, took up arms for rural land rights.

The violence drew in various rebel and paramilitary forces and drug gangs as well as state forces.

The conflict has left at least 260,000 people dead and displaced more than seven million, according to the authorities.

Colombian President Juan Manuel Santos was honoured with the 2016 Nobel Peace Prize in Oslo due to his efforts to end the conflict. In February, Bogota also began talks with the National Liberation Army (ELN, Guevarist), the country's last active guerrilla movement with some 1,500 fighters in hopes of achieving "complete peace".