Two decades after it started spraying coca fields with herbicide, Colombia is torn between continuing to wage a US-sponsored war on drugs from the sky and mounting fears of health risks.

Launched in 1994, the spraying program was long treated as sacrosanct by Colombian officials, who gladly accepted billions of dollars in funding from Washington and succeeded in slashing the cocaine production that has fueled the country's five-decade civil war.

But since the World Health Organization warned last month that glyphosate, the active ingredient in the herbicide, is "probably carcinogenic," infighting has broken out in President Juan Manuel Santos's cabinet over whether to continue the air war on coca, the raw ingredient for cocaine.

Health Minister Alejandro Gaviria said last week that Colombia should "immediately suspend" spraying -- a move vehemently opposed by Defense Minister Juan Carlos Pinzon, who said it would "give criminals the upper hand."

The row erupted just as US Deputy Secretary of State Antony Blinken paid a visit to Colombia, which the United States sees as one of its closes allies in the region.

The South American country has received $9 billion in US funding since 1999 under "Plan Colombia," a military and economic cooperation program aimed at fighting drug trafficking and the long-running insurgencies by the Revolutionary Armed Forces of Colombia (FARC) and the National Liberation Army (ELN).

Aerial spraying, carried out with American planes and pilots, is a key component of the plan. Blinken urged Colombia to continue using glyphosate, "the most efficient way to fight illegal crops."

He said the chemical -- sold by US biotechnology company Monsanto under the brand name Roundup -- was an agricultural product in widespread use in the United States and Europe.

"Glyphosate is used in every state in my country, and believe me, we would have taken measures if there were any problem with it," he told El Tiempo newspaper.

- 'High collateral cost' -

Not everyone agrees.

Daniel Mejia, the head of Colombia's Center for Research on Security and Drugs (CESED), called for a moratorium on glyphosate.

"We carried out a study that showed fumigating caused dermatological and respiratory problems and provoked miscarriages," he told AFP.

Mejia said aerial spraying has "little effect" anyway, because it only achieves results on three percent of the surface area treated.

"That doesn't justify such a high collateral cost to people's health," he said, urging authorities to focus on clandestine drug laboratories and smuggling routes instead.

But proponents say aerial spraying works.

Thanks in part to the program, Colombia has succeeded in reducing its coca fields from more than 140,000 hectares (346,000 acres) in 2001 to 48,000 hectares in 2013, according to the United Nations Office on Drugs and Crime.

It recently lost its long-standing title as the world's top cocaine producer to neighboring Peru.

Less cocaine means less drug money fueling the long-running conflict that has drawn in a string of leftist guerrilla groups, right-wing paramilitaries and drug traffickers.

The government has mainly used aerial spraying in the south of the country -- a stronghold for the FARC, the largest guerrilla group, which authorities accuse of financing itself with drug trafficking.

The FARC, which was founded in 1964 in the aftermath of a peasant uprising, says it is fighting for the rights of rural communities where coca farming is sometimes the only way to make a living.

The FARC and the government have been holding peace talks since 2012, and in May last year reached a deal that aims to eliminate illegal drugs by supporting alternative crops and giving farmers incentives to voluntarily destroy their coca plants.

But a peace accord has remained elusive, and both sides say the drug plan is contingent on reaching a final deal.

In the meantime, President Santos has side-stepped the row over the aerial spraying.

His staff said the final authority on the matter is the National Narcotics Council, which falls under the Justice Ministry.