At least 30 people were killed in an air strike during a concert held by an ethnic minority group in conflict with the ruling military, opposition groups, residents and media said on Monday.

The air strike late on Sunday in the northern state of Kachin, which the military has yet to confirm took place, killed civilians, prominent local singers and officers of the Kachin Independence Army (KIA), media reported, citing witnesses who said the attack was carried out by three jets.

A spokesperson for the junta could not immediately be reached for comment and Reuters could not immediately verify the details of the reports, which said the strike happened in the A Nang Pa region of Hpakant township.

Myanmar has been gripped by violence since the army overthrew an elected government early last year. Opposition movements, some of them armed, have since emerged across the country, which the military has countered with lethal force.

KIA spokesperson Naw Bu said the attack targeted celebrations of the 62nd anniversary of the formation of the Kachin army's political wing, the Kachin Independence Organisation (KIO).

"The air raid was intentional. The KIA/KIO strongly condemns this. This is quite a wicked act which can be also considered war crimes," he said by phone, adding his organisation could not yet confirm the death toll.

Since the coup, open conflict has resumed between the Myanmar army and bitter rival the KIA, which has been fighting for greater autonomy for the Kachin people for six decades and has voiced support for the anti-junta resistance.

The shadow National Unity Government (NUG), an alliance of anti-military groups, said it was greatly saddened by the loss of life and urged the United Nations and international community to intervene and urgently stop the atrocities.

"The terrorist military has deliberately committed another mass killing with aerial bombardments by targeting a large public concert," the NUG said in a statement.

"The act of the terrorist military clearly violates international laws."

The United Nations has repeatedly condemned Myanmar's military, accusing it of crimes against humanity over the deaths of civilians. The junta says its operations are targeting "terrorists".