At least 70 people were killed and 30 injured during an attack near Petite-Riviere in Haiti's breadbasket Artibonite region, a human rights group said on Monday, significantly higher than official estimates which put the death toll at around 16.

Residents and officials told local media the attack began in the early hours Sunday in rural communities around Jean-Denis, and continued into the early hours of Monday, with gang members storming the area and setting homes on fire.

The Defenseurs Plus rights group said it estimated 6,000 people had been displaced by the violence. The U.N. estimated that more than 2,000 people left their homes in the days prior following raids by armed gangs nearby.

A spokesperson for the U.N. Secretary-General told a press briefing the organization's office in Haiti, BINUH, was closely monitoring the events and that estimates ranged from 10 to 80 people killed. He called for a thorough investigation.

"The lack of a security response and the abandonment of Artibonite to armed groups demonstrate a complete abdication of responsibility by the authorities," Defenseurs Plus said in a joint statement with the Collective to Save the Artibonite.

An audio message circulated on social media was attributed to Gran Grif leader Luckson Elan. In the message, Elan appears to say the attack was retaliation for attacks on their base in Savien by a rival armed group.

Haiti's National Police said it deployed three armored vehicles, which were slowed by holes that gang members dug in the road. Officials said the armed group was fleeing the area when police arrived, and several houses were already burned down.

The injured were then taken to a local hospital and the dead to two morgues, said police, adding they have launched an operation to track down the gang members who fled.

Defenseurs Plus estimated 50 houses were burned down.

This month, the U.S. offered a reward of up to $3 million for information on their financial activities.

More than 1.4 million people – around 12% of the Caribbean's most populous nation – have been displaced by the conflict with armed gangs, worsening an economic crisis and access to food.

Reporting by Steven Aristil in Port-au-Prince and Natalia Siniawski in Mexico City; Editing by Sarah Morland, Brendan O'Boyle and David Gregorio (Reuters)