The states were created in accords drawn with the government of Prime Minister Rajiv Gandhi in an attempt to broker a solution to years of rebel insurgency by groups seeking independence from India.
Several of Assam’s borders with other states are also disputed.
The episode began Monday morning, when police officers from Assam state seized a newly constructed Mizoram police post, according to Mizoram officials. The Assam police said that the post was illegal because it fell within Assam’s borders, and that troops had taken it over in protest.
After the Assam police seized the Mizoram police post, officials from both sides tried to ease tensions.
But by Monday afternoon, hundreds of people, many Indigenous Mizo, from villages on the outskirts of the Singla Forest Reserve had joined the standoff, throwing stones and firing guns. At the same time, Himanta Biswa Sarma, the chief minister of Assam, and Zoramthanga, the chief minister of Mizoram, traded barbs on social media, tagging Prime Minister Narendra Modi’s office and Mr. Shah.
“Assam police used cane charging and tear gas smoke,” Vanlalfaka Ralte, the Kolasib district police chief, said.
“Assam side started firing on the Mizoram side,” Mr. Ralte said, “Then my troops replied.”
The Assam police had a different account. They said after they seized the post, Mizo civilians attacked them.
Afterward, Assam police officials said, Mizoram police opened fire on Assam police and civilians using automatic weapons including light machine guns, which are used in military combat.