Several more people have been sentenced to death "in secretive courts in violation of basic principles of fair trial", the UN human rights chief said.
Military leaders are releasing thousands, including an ex-British envoy, a Japanese journalist and an Australian economics adviser from prison, officials and local media say.
Several representatives of the Association of Southeast Asian Nations blame Myanmar's military rulers for failing to implement a peace plan agreed upon jointly last year.
Some children were killed on the spot by the shooting, while others died after troops entered the village in the central Sagaing region, according to local media reports.
Since seizing power, Myanmar's military government has detained thousands of pro-democracy protesters, with many facing charges that rights groups have decried as politically motivated.
Cambodia’s Foreign Minister Prak Sokhonn, speaking in his capacity as special envoy to Myanmar of the 10-member ASEAN, described the executions of Myanmar dissidents as a “setback” to his mediation.
Film-maker Kubota, 26, was detained last week near an anti-government rally in Yangon along with two Myanmar citizens.
Myanmar carries out its first executions in nearly 50 years, hanging a former lawmaker, a democracy activist and two men accused of violence after the country's takeover by the military last year.
The junta has sentenced dozens of anti-coup activists to death as part of its crackdown on dissent after seizing power last year, but Myanmar has not carried out an execution for decades.
Opponents of the military rule have marked the one-year anniversary of the army’s seizure of power with a nationwide strike to show their strength and solidarity amid international concerns.
UN human rights chief Michelle Bachelet said the world's response has so far been ineffective and delayed, calling on governments and businesses to urgently act to "commensurate to the magnitude of the crisis".
The United Nations estimates almost 90,000 people have fled after intense fighting in Loikaw town in eastern Myanmar's Kayah state last week.
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