Myanmar security forces backed by armoured vehicles clashed on Tuesday with a newly formed guerrilla group in the second biggest city Mandalay, army-run media, the militias and a witness said, resulting in at least two casualties.

Since the army seized power on Feb. 1 and removed Aung San Suu Kyi's elected government, troops have put down pro-democracy demonstrations and strikes and killed or arrested hundreds of protesters.

In response, people's defence forces have sprung up across the Southeast Asian country to take on security forces.

Until now, fighting involving lightly armed militias has been mainly in small towns and rural areas, but a group claiming to be Mandalay's new People's Defence Force said its members responded after the army raided one of its bases.

"The fight has started. There will be more fights," a militiaman identified as Captain Tun Tauk Naing said by telephone.

The sound of repeated gunfire could be heard in video footage taken by a resident in Mandalay, a hotspot for anti-coup protests.

Army-owned Myawaddy Television said on its Telegram message channel that security forces raided a house and "armed terrorists" fought back with small weapons and bombs.

It said four were killed and eight arrested and some members of the security forces were seriously hurt.

The junta typically describes its opponents as "terrorists".

Local news site Myanmar Now said about 20 soldiers conducted the raid, sparking a gunfight, and three armoured vehicles were deployed.

Another official from the militia group told the Mizzima news portal that six of its members had been arrested and two soldiers were killed.

An activist in Mandalay told Reuters he heard gunfire and saw about 10 armoured vehicles. "We are all afraid, but at least we know we have the support of the nation," he said. "Everyone in Myanmar knows the situation in Mandalay now."

A small group of demonstrators was seen rallying behind the Mandalay militias, carrying banners and making three-finger gestures symbolising resistance to military rule.

A spokesman for the junta did not answer calls seeking comment. State-run MRTV did not report the Mandalay unrest during its nightly newscast.

The U.S. Embassy in Myanmar said on Twitter that it was tracking reports of fighting in Mandalay and urgently called for a cessation of violence.

The military has used artillery and air strikes in response to guerrilla attacks on soldiers elsewhere in Myanmar, which have led to casualties on both sides and an exodus of tens of thousands of people.

The United Nations General Assembly called on Friday for a stop to the arms flow to Myanmar and urged the military to respect the outcome of a November election and release political detainees, including Suu Kyi.

Suu Kyi, 76, faces multiple charges including incitement, corruption and official secrets breaches. She appeared in court for her trial on Tuesday and was in normal health, her lawyer said. Her lawyers have said the charges are baseless.