BOGOTA - The United Nations denounced Colombia's anti-narcotics police on Monday for firing on a humanitarian mission it led to investigate the murder of several coca growers during clashes with security forces.

The mission, comprised of observers from the U.N. as well as the Organization of American States and church groups, were attempting Sunday to reach a remote outpost where the killings took place when they were stopped in their tracks by four exploding stun grenades as well as teargas and the sound of gunfire, according to the U.N.'s human rights office in Colombia.

The confusing incident is the latest embarrassment for President Juan Manuel Santos' government as it struggles to implement a peace deal with the Revolutionary Armed Forces of Colombia while trying to get a grip on booming coca production dominated by criminal gangs filling the void left by retreating rebels.

The problems in the turbulent municipality of Tumaco began on Thursday when several hundred farmers clashed with police as they attempted to eradicate illegal coca crops. At least six farmers were killed.

Initially the government blamed the murders on a dissident faction of the now demobilized FARC, saying they had fired homemade mortars on the very same farmers the guerrillas had ordered to confront police in order to protect their coca crops.

But eyewitness accounts verified by Colombia's ombudsman's office point to the police as the ones behind the slayings. As a result the government has since backtracked and on Monday the police suspended four officers accused of opening gunfire as a fuller investigation is carried out.

The U.N. human rights office in a statement expressed alarm that police with whom it had been in constant communication and authorized their movements had impeded their probe.

Vice-President Oscar Naranjo met with representatives of the U.N. and apologized for the police officers' behaviour, saying they had acted "irregularly."

Despite the in tragedy in Tumaco, Santos said security forces won't ease their campaign to root out drug trafficking in long-neglected rural areas.

"We won't permit lawless organizations to intimidate or pressure communities," Santos said at an event in a former FARC stronghold to roll out benefits for hundreds of towns hard hit by the country's half-century conflict. "We won't lower our guard against drug-trafficking, which was the fuel of so many years of violence."

News from © The Associated Press, 2017