At least 18 people have been killed and dozens wounded in two separate attacks in Colombia, deepening the country's most serious security crisis in decades.

Six people died and more than 60 were injured after a car bomb exploded on a busy street in the western city of Cali in Colombia, according to authorities.

Earlier on Thursday, a separate drone attack against a police helicopter killed at least 12 people in a rural area outside the northwestern city of Medellin.

The attacks, attributed to different dissident factions of the now defunct Revolutionary Armed Forces of Colombia (Farc) group, pose fresh challenges to Colombia's fragile peace processes ahead of elections next year.

Alejandro Eder, the mayor of Cali, ordered martial law for the country's third most populous city. He also announced a temporary ban on large trucks entering the city and called on the public to report information about the incident for a $10,000 reward.

In the wake of both attacks, the president and the military leadership announced they would lead a security council meeting to "define additional protection measures" for citizens.

"The state will not yield to terrorism. These crimes will be pursued and punished with the full force of the law," the Ministry of Defence said on social media.

According to eyewitnesses, the car bomb in Cali targeted the Marco Fidel Suarez Military Aviation School, killing civilians in the street and damaging many houses.

"There was a thunderous sound of something exploding near the air base," an eyewitness told AFP news agency.

Defence Minister Pedro Sánchez called the blast a "terrorist attack" and blamed "the narco cartel alias Mordisco" - referring to Farc guerrilla leader Ivan Mordisco.

"This cowardly attack against civilians is a desperate reaction to the loss of control over drug trafficking in Valle del Cauca, Cauca, and Nariño, where the Public Force has neutralized much of this threat," he said on social media.

Addressing the separate attack on a police helicopter, President Gustavo Petro said the aircraft was on a mission to eradicate coca leaf crops - a main ingredient in cocaine.

The helicopter crashed to the ground after being hit by a drone, killing the 12 officers on board.

Images circulating on social media showed thick plumes of black smoke billowing in a forested area of Amalfi in the country's north.

Sánchez said the attack was perpetrated by the EMC guerrilla group, the largest offshoot of Farc.

Colombia has experienced a rise in violence in recent months involving clashes between security forces and dissident rebels, paramilitaries or drug gangs.

Drone attacks have also become increasingly common in recent years: in 2024, 115 such attacks were recorded in the country, most of them carried out by illegal armed groups.

Last week, three soldiers were killed in a drone attack in the country's south-west, where explosive devices were dropped on members of the navy and army who were manning a checkpoint.