(CNN) Dozens of people have reportedly been killed in military airstrikes at a celebratory event in Myanmar's mountainous Kachin state on Sunday, drawing international condemnation of the junta that seized power in the country more than a year and a half ago.
Victims had been attending an event organized by the Kachin Independence Army (KIA) to mark the 62nd anniversary of the armed ethnic rebel group's political wing, the Kachin Independence Organization (KIO).
Col. Naw Bu, a spokesman for the Kachin Independence Organization (KIO), told CNN 62 people had been killed in the airstrikes and 60 people remain injured. Many of the injured have burns and shrapnel wounds.
CNN cannot independently verify the number of reported deaths.
KIO General Secretary La Nan said the event the victims were attending, which included musical performances, was one of the group's most significant annual festivities, with "hundreds, if not thousands" in attendance including artists, business owners and elders. Many had traveled from across the state to attend, he said.
"We understand the intention of (the airstrikes) was largely to inflict chaos and massive pain to the public, in a large volume and with as much damage as they could inflict," La Nan said.
The military junta, which overthrew the government in a bloody coup last February, claimed on Monday that reports of civilian deaths from the airstrikes were "fake news."
It claimed the airstrikes had targeted the KIA's military base, in response to the group's earlier raids and attacks on passenger vessels along the Irrawaddy River. It also claimed it had followed international conventions "so as to ensure peace and stability of the region."
La Nan refuted the junta's claim, saying the celebration had been held in the A Nang Pa region -- a small area where travelers often stop by a market. It's "nowhere close to military installations," he said.
Though KIO personnel were in attendance, "they were not there as military personnel but as entertainers," helping welcome guests and performing, he added.
Since the coup, rights groups and observers say freedoms and rights in Myanmar have deteriorated; state executions have returned and the number of documented violent attacks by the army on schools has surged.
Numerous armed rebel groups have emerged, while millions of others continue resisting the junta's rule through strikes, boycotts and other forms of civil disobedience.
Myanmar's shadow government, the National Unity Government -- a group of ousted lawmakers, coup opponents and ethnic minority group representatives -- condemned the attack in a statement on Monday, saying the military had "deliberately committed another mass killing."
The attack "clearly violates international laws as the provisions of the Geneva Conventions," it said in the statement, urging the international community and United Nations to "take effective actions urgently."
The NUG operates undercover or through members abroad, seeking to gain recognition as the legitimate government of Myanmar.
The attack on Sunday drew international condemnation, with the United Nations saying it was concerned over reports of more than 100 civilians impacted.
"While the UN continues to verify the details of this attack, we offer our deepest condolences to the families and friends of all those who were killed or injured. The UN calls for those injured to be availed urgent medical treatment, as needed," it said in a statement on Monday.
It added that the military's "excessive and disproportionate" use of force against unarmed civilians was "unacceptable," and called on those responsible to be held to account.
Military spokesperson Brig Gen Zaw Min Tun rejected the reports in comments made to CNN Wednesday.
"We knew that it was a den where enemies and terrorist were hiding. So, we launched a necessary military operation," he said. "Throughout history and till now, Tatmnadaw has never attacked civilians, and we don't need to clarify about the matter in connection with fake news."
La Nan, the KIO official, said the military had sealed off the roads surrounding A Nang Pa after the attack, imposed an internet and telecommunications blackout, and deployed plainclothes officers to local hospitals -- meaning victims of the attack have little to no access to medical care.
"They are taking refuge in nearby makeshift clinics and rudimentary medical facilities in the mining area. Most of their relatives are very worried because there is very little access to medicine," he said, calling it a "deliberate blockade."
The KIO and local community is now trying to recover all the victims, and "have a proper burial according to our traditions and our religious rituals," he said -- adding that about 10 bodies were beyond identification.
CNN cannot independently verify the current situation.
On Monday, the ambassadors of Australia, Canada, Czech Republic, Denmark, Finland, France, Germany, Italy, Netherlands, Spain, Norway, Switzerland, the United Kingdom and the United States issued a joint statement condemning the strike.
"This attack underscores the military regime's responsibility for crisis and instability in Myanmar and the region and its disregard for its obligation to protect civilians and respect the principles and rules of international humanitarian law," the joint statement read.
Non-profit organization Amnesty International said in a statement the military's actions -- including executing pro-democracy activists, jailing journalists and targeting civilians -- have been allowed to continue "in the face of an ineffective international response."
"As officials and leaders from the Association of Southeast Asian Nations (ASEAN) prepare to host high-level meetings in the coming weeks, this attack highlights the need to overhaul the approach to the crisis in Myanmar," the statement said, urging ASEAN leaders to take action when they meet for their annual summit in November.