Colombia's ELN rebel group was responsible for the car bomb attack against a police academy that killed at least 21 and injured dozens, the government said on Friday, making it unlikely that peace talks with the armed group can restart any time soon.

In Thursday's attack, a vehicle driven by ELN explosives expert Jose Aldemar Rojas broke through checkpoints onto the grounds of the General Santander School before it detonated, shattering windows of apartments nearby, Defence Minister Guillermo Botero said.

"A terrorist act by the ELN took those lives," he told reporters in Bogota.

The National Liberation Army (ELN), made up of some 2,000 fighters and considered a terrorist organisation by the United States and European Union, began peace talks with the government of former President Juan Manuel Santos in early 2017, but they have been put on hold. Right-wing President Ivan Duque has demanded the ELN free its 17 hostages and stops attacks before new talks can begin. The ELN has dismissed the conditions as unacceptable.

The bombing heightens fears that a new wave of attacks could be launched in the Andean nation as a reaction to the right-wing president's stance.

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Duque, who took office in August and is supported by hard-line former President Alvaro Uribe, has pledged to toughen offensives against the group and drug-trafficking crime gangs that continue to kill and kidnap civilians.

The government in November asked Cuba - where talks were taking place - to capture ELN rebel commander Nicolas Rodriguez and provide information about the presence of other ELN commanders in Cuban territory.

It also asked Havana to act on an Interpol Red Notice on Rodriguez.

80kg of explosives

Rojas, known by his war alias El Mocho or Kiko, had lost his hand in bomb-making activity and travelled frequently across the border to Venezuela, Botero said.

The attack was planned over about 10 months and Rojas was killed in the blast, he said.

Duque called the explosion a "crazy terrorist act" against unarmed cadets.

"We will not rest until we capture and bring to justice the terrorists involved," Duque said late on Thursday. "I tell the criminals that social repudiation awaits them, the rejection of all Colombians and the international community."

The vehicle, a grey Nissan Patrol 4X4, was carrying an estimated 80kg of the highly explosive pentolite, which has been used in the past by Colombian rebels, Attorney General Nestor Humberto Martinez said on Friday.

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Car bombs were frequently used in Colombia during decades of civil war between the government and various leftist rebel groups, as well as in violence involving the Medellin drug cartel led by the late drug lord Pablo Escobar.

The worst of the war, which killed some 260,000 and left millions displaced, ended when the government of Juan Manuel Santos reached a peace agreement with the Revolutionary Armed Forces of Colombia (FARC) in 2016.

The last major attack was in January 2018 when the ELN, the biggest rebel group that remains active, detonated a bomb in the northern port city of Barranquilla, killing five police officers and injuring dozens.

The ELN was established in 1964 by Catholic priests to fight for land rights and the protection of poor rural communities.