Vietnam has listed two political groups operating in the United States as "terrorist organisations", accusing them of orchestrating attacks and promoting a secessionist agenda, its internal security agency said on Wednesday.

The groups are the North Carolina-headquartered Montagnard Support Group Inc (MSGI) and Montagnard Stand for Justice (MSFJ), which was established in Thailand, the Ministry of Public Security said in a statement.

Both operate in the United States and are accused of involvement in deadly attacks in the Central Highlands region in June 2023 that killed nine people, including four policemen.

The U.S. embassy in Hanoi and the two groups did not immediately respond to separate Reuters requests for comment.

Montagnards, or "mountain people" in French, are an ethnic minority from Vietnam's Central Highlands. Many are Protestant Christians who sided with the United States during the Vietnam War.

The communist government has long been sensitive to the activities of overseas-based dissident groups led by members of Vietnam's vast diaspora, typically calling them "reactionaries". It also retains tight media censorship and tolerates little criticism.

It has listed as terrorist groups several organisations that have pledged allegiance to the defunct state of South Vietnam, which ceased to exist when the Communist North won the Vietnam War in 1975.

The two groups listed on Wednesday had recruited ethnic minority people in Vietnam, trained them and instructed them to "carry out terrorist activities, incite protests, kill officials and civilians, sabotage state assets and try to establish their own states", the ministry statement said.

"Anyone joining the groups or receiving funding from the groups would be charged with terrorism and be dealt with," the statement said.

Separately, the Human Right Watch on Wednesday said Vietnamese authorities had over the past week arrested three prominent activists on charges of conducting propaganda against the state, and called on the government to immediately release them.

"These three activists are not guilty of anything except exercising their basic rights to freedom of speech," said Phil Robertson, deputy Asia director at Human Rights Watch.