Naypyidaw: Thousands of people from Myanmar crossed into India after some ethnic minority insurgent groups attacked security posts in Myanmar on Monday leading to a fresh fight erupting on two new fronts, Reuters reported.

This comes as Myanmar's military junta is facing its biggest test since taking power in a 2021 coup after three ethnic minority forces launched a coordinated offensive in late October, capturing some towns and military posts.

Last week, the military-installed president said that Myanmar was at risk of "breaking apart" because of an ineffective response to the rebellion.

The military generals say that they are fighting "terrorists".

One of the three allied insurgent groups, the Arakan Army (AA), which is fighting for greater autonomy in Rakhine State in western Myanmar, seized posts in the Rathedaung and Minbya areas, AA spokesman Khine Thu Kha said.

"We have conquered some posts and fighting is continuing in some other places," Reuters quoted Khine Thu Kha as saying.

A resident of Rathedaung said gunfire was heard before dawn on Monday followed by hours of artillery bombardment, with the military seen blocking entrances to the area and reinforcing administrative buildings.

Notably, about 5,000 people from Myanmar crossed into India's Mizoram state to seek shelter from the fighting, Reuters reported citing James Lalrinchhana, the Deputy Commissioner of a district on the Myanmar border.

On the other hand, Chin State, which had been largely peaceful for years, saw fierce fighting after the 2021 coup with thousands of residents taking up arms against the military administration.

According to the report, the new combat will be another blow for the junta which is increasingly stretched amid mounting armed opposition, fuelled by anger over the coup and crackdown.

The coordinated anti-junta offensive launched on October 27 in Shan State in the northeast has seen several towns and more than 100 military posts seized near the border with China.

Assaults on urban centres have also taken place in the Sagaing region in central Myanmar, to the west of Shan State, while the conflict in neighbouring Kayah State to the south led to the crash on Saturday of a fighter jet, the report stated.

The rebels said they shot the aircraft down while the military said it had a technical fault.

Richard Horsey, senior Myanmar Adviser for the Crisis Group think tank, said the military had experience fighting in Rakhine State but could struggle as enemy forces probe for weaknesses in multiple areas, Reuters reported.

"If combat persists, it will open a significant new front for the regime, which is already overstretched," he said.

"It will be hard for the regime to focus their efforts across all fronts," Horsey added.