Nguyen Quang Khanh was released Wednesday after nine days in detention, the US based Viet Tan (Vietnam Reform Party) said in a statement.

It has identified Khanh as a member of Viet Tan, along with Nguyen Ly Trong, who was released and returned to the US earlier this week.

Viet Tan said another member of the group, Jennifer Truong, remains in detention following the March 14 demonstration in Ho Chi Minh City against government corruption and illegal land expropriation.

All three are of Vietnamese descent.

In Washington, a State Department spokesman said on Tuesday -- before Khanh's release that US diplomats had been informed by Vietnam that two Americans were detained.

"The US Consulate General in Ho Chi Minh City is seeking additional information and has requested consular access," the spokesman said.

Vietnamese officials have not responded to a request for comment.

Authorities are also holding a French Vietnamese citizen, Pham Minh Hoang, who was accused in September of being a Viet Tan member and of plotting to overthrow the government.

Vietnam is a one-party communist state that labels Viet Tan a "terrorist organisation".