A long-running interstate border conflict between two northeast India states has erupted in violence.

Six Assam cops were killed and more than 80 people, including officials, were injured, when Mizoram police opened fire.

Assam and Mizoram share a 164 kilometer (101 mile) border, and both states contest its boundaries. The two sides have frequently, and sometimes violently, sparred over it. However, this was the first time police officers in the two states fired at each other in a direct encounter.

Since 1994, the federal government has attempted to mediate a truce between the states but has failed to achieve a resolution.

Home Minister Amit Shah reportedly spoke with the chief ministers of Assam and Mizoram, Himanta Biswa Sarma and Zoramthanga, on Monday, urging them to find a "amicable solution" to the border dispute.

Assam's chief minister, Sarma, announced on Twitter that members of the state police had been killed by Mizoram forces "while defending the constitutional boundary."

Assam police also claimed that "miscreants" from Mizoram pelted them with stones and attacked government officials.

However, Mizoram authorities denied this.

According to the state's home minister, Lalchamliana, Mizoram cops spontaneously fired back at Assam police officers after they "forcibly crossed" a post manned by the Central Reserve Police Force.

The federal government has stationed the police force as a "neutral force" along the disputed border.

Meanwhile, chief ministers Zoramthanga and Sarma argued on Twitter, blaming one other for the violence and pleading for federal intervention.

Mizoram was a part of Assam until 1972, when it was divided. In 1986, it became a separate state.

Mizoram claims that the area claimed by Assam has been used by its people for more than a century, but the Assamese say that a substantial portion of land has been forcibly stolen.