Colombian President Juan Manuel Santos expressed regret Monday after police fired shots and hurled grenades at a humanitarian delegation that included representatives of the United Nations and the Organization of American States (OAS).

"The incident yesterday is a regrettable deed. Our government always values the work of monitoring authorities and humanitarian missions such as the one that was attacked yesterday," he said during a public event in the central province of Meta.

Sunday's episode took place in a rural area near Tumaco, a port city in the southwestern province of Nariño.

The delegation comprised members of the Colombian NGO Conpaz and of the OAS Mission to Support the Peace Process, UN personnel, and journalists from El Espectador and El Tiempo and the newsweekly Semana.

The mission set out to reach the remote village of Tandil, where six people were killed and 20 others wounded last Thursday while protesting against the government's push to eradicate coca, the raw material of cocaine.

While initial reports attributed the killings to renegade FARC rebels who reject the peace accord the insurgency signed last year with the Santos government, some grassroots groups says it was police who did the shooting and four officers have been suspended.

Santos, who was awarded the 2016 Nobel Peace Prize for ending the decades-long conflict with the FARC, said Monday that he ordered the police and the armed forces "to ensure total cooperation with the (judicial and administrative) investigations to create absolute clarity about all of these events and determine responsibilities."

"The actions of the public forces against criminal bands and in the eradication of illegal crops must be decisive, but always with the strictest respect for human rights," the president said, though adding that the government would not allow "organizations outside the law to intimidate or pressure communities."

Tumaco, which has Colombia's largest concentration of coca plantations and is the scene of violent turf battles among criminal outfits, "is a priority region for the government and for the state in the construction of peace," Santos said.

Even as the president sought to address the events in Nariño, news came of a journalist killed Sunday during a clash between indigenous people and police in neighboring Cauca province.

The National Indigenous Organization of Colombia (ONIC) identified the victim as Efigenia Vasquez Astudillo and said she worked for Renacer Kokonuko, a community radio station operated by the Kokonuko people.

Vasquez died at San Jose Hospital in Popayan, the provincial capital.

The reporter was hurt when police forcibly evicted indigenous people who were blocking access to a tourist spot known as Aguas Tibias.