Kachin guerrillas have attacked a police outpost in northern Myanmar, local media reported, in an indication of the deepening involvement of ethnic minority armed forces in the country's popular movement seeking to unseat the junta that seized power in February.

The action early on Wednesday follows conflict in eastern Myanmar, where Karen guerrillas seized an army outpost on Saturday and Myanmar’s military followed with air strikes that killed about 10 villagers and drove thousands more across the border into Thailand.

After the air strikes, the Karen National Union issued a statement from one of its armed units saying Myanmar military "ground troops are advancing into our territories from all fronts" and it may have to respond. 

The KNU is the main political body representing the Karen minority.

READ MORE: Myanmar activists call for show of defiance on Armed Forces Day

Rebels as counterweight to regime?

The conflict in eastern Myanmar expanded the crisis regionally, as an estimated 3,000 Karen took temporary shelter in neighbouring Thailand. 

Thai authorities said on Wednesday only about 200 remained in the country and were preparing to go back across the border.

Leaders of the protest movement against the military's February 1 ouster of the elected government of Aung San Suu Kyi have been seeking an alliance with the ethnic minorities' armed groups to boost pressure on the junta. 

They would like them to form what they are calling a federal army as a counterweight to the regime's armed forces.

It is unclear what, if any, progress has been made toward that goal, though several of the major groups — including the Kachin, the Karen and the Rakhines' Arakan Army in western Myanmar — have publicly denounced the coup and said they will defend protesters in the territory they control.

READ MORE: Thousands flee to Thailand as Myanmar jets hit Karen villages

Rebels seek autonomy since decades

More than a dozen of Myanmar's ethnic minorities have been seeking greater autonomy for decades, going through cycles of armed conflict and uneasy ceasefires.

The Kachin Independence Army, the armed wing of the Kachin Independence Organization, attacked a police station in Kachin state's Shwegu township before dawn in Wednesday, local outlets The 74 Media and Bhamo Platform reported. 

The attackers were reported to have seized weapons and supplies and wounded one police officer.

The Kachin have staged a series of attacks on government forces since the coup, saying the latest round of fighting was triggered by government assaults on four Kachin outposts. 

After one Kachin attack in mid-March, the military retaliated with a helicopter attack on a Kachin base.

READ MORE:Myanmar junta launches new air attacks as violence continues

Anti-coup protests 

Protests meanwhile continue in Myanmar's cities against the military takeover that reversed a decade of progress toward democracy in the Southeast Asian country that followed five decades of army rule.

Anti-coup protesters marched through an area of Yangon on Wednesday despite reduced numbers in the face of the ever-climbing death toll.

At least 521 protesters have been killed since the coup, according to Myanmar’s Assistance Association for Political Prisoners, which counts those it can document and says the actual toll is likely much higher. 

It says 2,608 people have been detained, a total that includes Suu Kyi.

The mainly young demonstrators processed through the city's Hlaing suburb, stopping to honour a protester killed in an earlier confrontation with security forces.

The US State Department on Tuesday ordered non-essential US diplomats and their families to leave Myanmar, expecting the protests to continue. 

The US earlier suspended a trade deal and imposed sanctions on junta leaders as well as restricted business with military holding companies.

READ MORE: Anti-coup protesters back on Myanmar streets after deadly day