"I am dismayed that despite appeals from across the world, the military conducted these executions with no regard for human rights," UN High Commissioner for Human Rights Michelle Bachelet said in a statement.

"This cruel and regressive step is an extension of the military's ongoing repressive campaign against its own people".

The four were executed for leading "brutal and inhumane terror acts", the Global New Light of Myanmar newspaper said, without saying when or how the men were killed.

The executions marked the country's first use of capital punishment in decades and sparked fears that more death sentences will be carried out.

The junta has sentenced dozens of anti-coup activists to death as part of its crackdown on dissent after seizing power last year.

Bachelet strongly condemned the fact that Myanmar had gone ahead with the executions despite repeated calls from the UN and the wider international community to refrain from doing so.

The executions, she said, "are cruel violations of the rights to life, liberty and security of a person, and fair trial guarantees".

She called for the "immediate release" of all political prisoners and urged Myanmar to "reinstate its de-facto moratorium on the use of the death penalty, as a step towards eventual abolition".

According to the UN, 117 people, including two children, have been sentenced to death since the February 2021 coup. Of those, 41 were sentenced in absentia.

Over 11,500 people remain in detention for opposing the coup, Monday's UN statement said.

"Most concerningly, over 30 percent of over 2,100 people killed since February 2021 have died in military custody -- most as a result of ill-treatment," Bachelet said.