A Mark Zuckerberg attempt to clear the air around Facebook isn't sitting well with civil society groups in Myanmar, where the social networking service has been blamed for fueling ethnic violence.

On Thursday, the groups released a letter slamming Zuckerberg for glossing over Facebook's problems in the country during an interview with Vox Media that was published earlier this week.

In the interview, Zuckerberg claimed his company was able to stop attempts over Facebook Messenger to incite violence between Myanmar's Muslim and Buddhist groups. "Now, in that case, our systems detect that that's going on. We stop those messages from going through," Zuckerberg said.

However, the civil society groups in Myanmar paint a different picture, and claim that Facebook failed to quickly notice the problem last September, when the incident occurred. At the time, bad actors had been encouraging violence over Facebook Messenger with content that managed to circulate for over four days, possibly reaching hundreds of thousands of people.

Only when the civil society groups alerted Facebook about the abuse did the social media giant finally intervene, their letter says.

"In your interview, you refer to your detection 'systems'. We believe your system, in this case, was us —and we were far from systematic," the letter adds.

Although Facebook eventually stopped the abuse, by then the offending messages had caused widespread fear and at least three violent incidents, according to the groups. Making matters worse is that Facebook is ill-equipped to stop future attempts to incite violence over the platform, they add.

"As far as we know, there are no Burmese speaking Facebook staff to whom Myanmar monitors can directly raise such cases," their letter reads. "We were lucky to have a confident English speaker who was connected enough to escalate the issue."

So far, Facebook hasn't commented on the letter. But criticism of the company's role in fanning ethnic violence in Myanmar is only growing. Last month, officials with the United Nations said that Facebook had become a tool to spread violent messages in the country.

"Hate speech and incitement to violence on social media is rampant, particularly on Facebook," said UN human rights investigator Marzuki Darusman. "To a large extent, it goes unchecked."

Facebook is well aware of the problems and trying to solve them, according to Zuckerberg. One company executive has even admitted that his teams are losing sleep over how the platform is igniting violence in the country.

The civil society groups in Myanmar say they want to work with Facebook to crack down on the abuse. But so far, they've only engaged with Facebook's policy team, and not the product or engineering divisions. Attempts at greater collaboration have also gone unanswered, the groups claim.

"The risk of Facebook content sparking open violence is arguably nowhere higher right now than in Myanmar," their letter adds.

The letter comes from six groups including the IT innovation lab Phandeeyar, the Myanmar ICT for Development Organization, Burma Monitor, the Center for Social Integrity, Equality Myanmar, and Myanmar Human Rights Educator Network.