The United States on Wednesday called on Vietnam to free three bloggers facing trial for propaganda against the state, voicing deep concern at the self-immolation of the mother of one of the trio.

The defendants were arrested after posting hundreds of political articles on the banned Vietnamese website "Free Journalists Club," as well as writing on their own blogs, in a case that has been raised by U.S. President Barack Obama.

If convicted they face up to 20 years in prison, according to their lawyers.

Dang Thi Kim Lieng, the mother of one of the three, died after setting herself ablaze on Monday in front of the local authorities' offices in Bac Lieu province, according to people close to the family.

"We were deeply concerned and saddened to hear of (Lieng's) self-immolation reportedly related to circumstances surrounding the detention of her daughter Ta Phong Tan," the US Embassy in Hanoi said in a statement.

"We call upon the Government of Vietnam to release all three immediately and, as President Obama said on World Press Freedom Day, 'take the necessary steps to create a society where journalists can operate freely and without fear'," the statement said.

The government has not responded to an AFP request for comment on the self-immolation and there has been no official confirmation of the incident by the authorities, nor have Vietnam's tightly-controlled state media reported it.

Tan, a 43-year-old Catholic former policewoman, used her blog to denounce corruption and injustice in Vietnam's legal system.

She was arrested on September 5, 2011 and has been held ever since.

She is due in court in Ho Chi Minh City next week alongside Phan Thanh Hai, who blogged on highly sensitive topics including territorial disputes with China, and Nguyen Van Hai, better known by his online alias Dieu Cay.

The hearing for the trio is set to start on August 7, Dieu Cay's lawyer Ha Huy Son told AFP previously.

[image via Agence France-Presse]