The Assam Police targeted vehicles on the highway, used batons and tear gas on the locals, Mizoram said.
Gunshots ringing out on the highway, puffs of smoke, a road littered with stones and tear gas canisters and upturned vehicles of the roadside -- the six-hour violence at the Assam-Mizoram border at Kolasib left the area looking like a battlefield and five men of the Assam Police dead and more than 50 people, including several policemen, injured.
Videos of the violence were tweeted by the Chief Ministers of both states. More made the rounds on social media, indicating that Monday's violence in the area known for its volatility, has surpassed anything that took place in the recent times.
In a statement, Assam accused Mizoram of building a road destroying a Reserve Forest area in Lailapur and setting up an armed camp at the spot.
The state said their police team was attacked by armed "miscreants" when they went to resolve the situation. The Mizoram Police supported the mob and fired tear gas shells on the Assam Police.
When the Assam Police requested the Mizoram police to control the mob, they said they were unable to do so.
"Horrifically even as even as SP Kolasib was in talks with Assam officers, Mizoram Police opened fire on Assam officials and civilians... with automatic weapons and LMGs (Light Machine Guns)," the Assam government said in a statement.
Mizoram said it was "unprovoked aggression" by the Assam Police, who went to Kolasib around 11 am, "forcibly" crossing the duty post manned by CRPF personnel and overrunning a duty post manned by Mizoram police personnel.
The Assam Police then targeted vehicles on the highway. When the locals came to inquire, they used batons and tear gas on them. The local police and a magistrate who went to meet them were rebuffed.
The "Assam side were adamant and unwilling to discuss," the Mizoram government said in a statement. "The confrontation continued and a volley of tear gas canisters and grenades were launched at Mizoram Police followed by firing from Assam side at around 4.50 pm," the statement read.
"Mizoram Police responded spontaneously by firing back at Assam Police in spite of the fact that SP, Kolasib District was still inside CRPF duty camp negotiating with the Assam Police authorities," the statement added.
Mizoram shares a 164.6 km borders with Assam that runs through three districts on each side -- Mizoram's Aizawl, Kolasib and Mamit and Assam's Cachar, Hailakandi and Karimganj districts -- and violence in the area has persisted for decades. Each side, residents and security forces, accuse the other of intrusion.
On Monday evening, Union Home Minister Amit Shah has spoken to Chief Ministers of both states and asked them to resolve the border issue, to which they have agreed sources said. Mizoram had already formed a committee to look into the issue.