The United States has condemned the conviction of seven Vietnamese land rights activists, including a church pastor, during a one-day closed trial in the communist country.

The defendants were convicted of attempting to overthrow the government and were sentenced to between two and eight years in prison during Monday's trial in the southern province of Ben Tre. They were also given between three to five years of probation, according to the official Vietnam News Agency.

Court officials declined to comment.

A U.S. Embassy spokesman in Hanoi said Tuesday that diplomats were denied access to the trial and that Washington had expressed concerns about the proceedings to senior Vietnamese officials.

"We are particularly concerned by reports that several of the defendants were denied access to legal representation prior to their trial," said spokesman Beau Miller.

"Freedom of assembly and expression are fundamental internationally recognized rights. No individual should be punished for peacefully exercising these rights," he added.

Prior to the trial, five members of the U.S. Congress sent a letter to Prime Minister Nguyen Tan Dung calling for the charges to be dropped.

The activists — who included Duong Kim Khai, a pastor at a Mennonite church that's unrecognized by the government — were lobbying for land rights in Ben Tre. They were arrested between August last year to early this year.

Demonstrations are routinely squashed by security forces in Vietnam, which does not tolerate any challenge to its one-party rule.

Khai and two other convicted activists are members of Viet Tan, a U.S.-based group also known as the Vietnam Reform Party, which is banned in Vietnam and considered a threat to the government.

"With these unjust sentences, the Hanoi regime has attempted to silence seven activists who spoke out against the regime's failings," according to a statement posted on Viet Tan's website.

The Vietnam news report said six of the seven defendants were accused of associating with Viet Tan and were sent to Thailand and Cambodia for training for overthrowing the government by nonviolent means.

The were also accused of instigating people to demonstrate and cause disturbances to oppose the government, it said.

"Their crimes are very serious, encroaching on national security and national interests," it quoted the indictment as saying.