Major Roberto Samcam, a retired Nicaraguan army officer in exile who fiercely criticised authoritarian President Daniel Ortega, was shot dead Thursday, his family and officials said.

Samcam, 66, was gunned down at his apartment building in San Jose, reportedly by men pretending to deliver a package.

"We condemn the murder of Roberto Samcam in the strongest terms, and welcome the swift action of Costa Rica, which, we trust, will expose the motivations behind this terrible act and bring justice to his family," said Jan-Michael Simon, chair of the UN Group of Human Rights Experts on Nicaragua.

In a report published in February, the group warned that the Nicaraguan government's actions against opponents, whether real or perceived, did not stop at the border.

"Nowhere in the world seems to be safe for Nicaraguans opposed to the government of Daniel Ortega and Rosario Murillo," said Reed Brody, one of the group members.

"Host countries should step up the protection of Nicaraguans living in their territory."

UN experts are appointed by the Human Rights Council but are independent figures and therefore do not speak in for the United Nations itself.

Samcam, who was a political analyst, had spoken out frequently against the government in Managua, which he fled in 2018 to live with his wife in Costa Rica.

That year, protests against Ortega's government were violently repressed, resulting in more than 300 deaths, according to the UN.

"Government tactics to control and silence dissenters abroad include threats, financial pressure, entry bans, the denial of consular services and passports, the arbitrary deprivation of nationality, harassment, surveillance and coercion via family members," said Peralta.

"The possibility that the murder of Roberto Samcam is part of this pattern should not be discarded."

Ortega, now 79, first served as president from 1985 to 1990 as a former guerrilla hero who had helped oust a brutal US-backed regime.

Returning to power in 2007, he became ever more authoritarian, according to observers, jailing hundreds of opponents, real and perceived, in recent years.

Thousands of Nicaraguans have fled into exile, and the regime is under US and EU sanctions.