Air strike during Myanmar concert kills at least 50

* 3 jets hit rebel group's anniversary event - reports
* Military fighting resistance on multiple fronts
* UN, western missions condemn attack
* Rebels, shadow govt accuse junta of war crimes

An air strike killed at least 50 people at a concert in Myanmar held by an ethnic minority group in conflict with the ruling military, opposition groups and media said on Monday, an attack condemned by the United Nations and western embassies.
The jet strike late on Sunday in the northern state of Kachin, which the military has yet to confirm, killed civilians, local singers and officers of the Kachin Independence Army (KIA), media reported, citing witnesses who said three planes carried out the attack.
A spokesperson for the junta could not immediately be reached for comment and state television made no mention of the incident in its nightly news bulletin on Monday.
It took place in the A Nang Pa region of Hpakant township and killed at least 50 people, BBC Burmese said, while news site Irrawaddy put the death toll at about 100. Reuters could not immediately verify the figures.
Myanmar has been gripped by fighting since the army overthrew an elected government early last year. Resistance movements, some armed, have 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 founding of the Kachin army's political wing, the Kachin Independence Organisation (KIO).
"This is quite a wicked act that can be also considered war crimes," he said by phone.
Since the coup, open conflict has resumed between the Myanmar army and bitter rival the KIA, which has been fighting on and off for six decades for greater autonomy for the Kachin people. The KIA has voiced support for the anti-junta resistance.
The UN in Myanmar said it was deeply concerned and saddened by reports of the attack.
"What would appear to be excessive and disproportionate use of force by security forces against unarmed civilians is unacceptable," it said in a statement.
In a joint statement, heads of diplomatic missions in Myanmar including Australia, Britain, the United States and European Union members said the attack "underscores the military regime's responsibility for crisis and instability ... and its disregard for its obligation to protect civilians."
Myanmar's shadow National Unity Government urged the UN and international community to intervene and urgently stop the atrocities.
The military has repeatedly lashed out at the UN for what it sees as interference in Myanmar's internal affairs and says its operations are targeting "terrorists".
Foreign ministers of the Southeast Asian bloc ASEAN are due to meet later this week to discuss the Myanmar crisis, its chair Cambodia said on Sunday.