Southeast Asian leaders on Tuesday began their annual summit without Myanmar, amid a diplomatic standoff over the military-ruled nation’s exclusion from the group’s meetings.

Myanmar skipped the summit in protest after the regional bloc — the Association of Southeast Asian Nations, known as Asean — shut out its top general from its meetings.

The group’s refusal to allow Senior Gen. Min Aung Hlaing to represent Myanmar at the summit was its harshest rebuke yet of the country’s military rulers since the generals ousted civilian leader Aung San Suu Kyi in February.

Brunei, who is this year’s chair of the 10-member bloc, invited Myanmar’s highest-ranking veteran diplomat, Chan Aye, as a “non-political” representative but she didn’t attend the meeting, two diplomats said. The diplomats requested anonymity as they are not authorized to speak to the media.

Myanmar’s Foreign Ministry vowed late Monday to challenge Asean’s unprecedented move to downgrade its participation in the three-day virtual summit, which is being held by video due to coronavirus concerns. The ministry said it will only accept representation by the country’s top general, who heads its government and ruling military council, or a ministerial-level representative.

The talks will be joined by other world leaders including U.S. President Joe Biden and the leaders of China and Russia, and are expected to spotlight Myanmar’s worsening crisis, as well as regional security and economic issues.

Biden’s participation will be the first time since 2017 that a U.S. leader has attended the Asean summit.

The military takeover in Myanmar triggered widespread protests and led to a violent crackdown by authorities. Almost 1,200 civilians are estimated to have been killed by security forces, though the government has claimed a lower death toll.

Asean’s sanctioning of Myanmar marked a shift from the bloc’s bedrock principles of non-interference in each other’s domestic affairs and decision by consensus. Myanmar cited the violation of those principles — enshrined in the group’s charter — when it rejected Asean’s ban on its military leader from the summit.

Myanmar’s absence at the summit followed the refusal of its military leaders to allow the bloc’s envoy, Brunei Second Foreign Minister Erywan Yusof, to meet with Suu Kyi and other civilian leaders detained since the military takeover.

Asean leaders called for an immediate end to violence in Myanmar in an emergency meeting in April and outlined a plan for dialogue between both civilian and military figures to be mediated by the bloc’s envoy. Myanmar’s Min Aung Hlaing attended the meeting.

Asean leaders are due to hold talks with their counterparts from China, South Korea and the U.S. later Tuesday.

Ahead of the talks, a senior U.S. official held a virtual meeting with two representatives of Myanmar’s political opposition National Unity Government, which views itself as a shadow government and had earlier sought to attend the Asean summit.

National Security Advisor Jake Sullivan “underscored U.S. continued support for the pro-democracy movement” in Myanmar during Monday’s meeting.

Sullivan expressed concern over the military’s brutal violence and said Washington will continue to seek the release of all those “unjustly detained,” including prominent pro-democracy activist Ko Jimmy, who was the latest to be held during a raid Saturday.