Nguyen Ngoc Nhu Quynh was convicted in June of spreading propaganda by distorting government policies and defaming the Communist regime.
Lawyer Nguyen Ha Luan said the People's High Court determined after a half-day hearing Thursday in south-central province of Khanh Hoa that the conviction was justified.
Quynh, a single mother of two young children, maintained she was innocent throughout the trial, Luan said. She admitted to have written the articles but said they did not constitute a crime, he said.
"Our view is that she did not commit a crime," Luan said, adding that the prosecution had failed to prove her guilt.
The 37-year-old blogger known by her pen name Me Nam, or "Mother Mushroom," co-founded a network of bloggers and is very popular in Vietnam. She has written about human rights and civilian deaths in police custody and the release of toxic chemicals by a Taiwanese-owned factory that killed thousands of fish in one of Vietnam's worst environmental disasters.
U.S. Charge d'Affaires Caryn McClelland said in a statement she was "deeply troubled" by the court's decision to uphold the sentence on a "vague" charge and called for the blogger's release.
"The United States calls on Vietnam to release Ms. Quynh and all prisoners of conscience immediately, and to allow all individuals in Vietnam to express their views freely and assemble peacefully without fear of retribution," she said.
McClelland said Quynh was one of six individuals convicted this year for exercising the right of fundamental freedom of expression, association and of peaceful assembly. "The trend of increased arrests, convictions, and harsh sentences of peaceful activists and students since early 2016 is deeply troubling," she said.
Vietnam's Foreign Ministry spokeswoman Le Thi Thu Hang told reporters at a regular press briefing Thursday that Quynh's appeals trial had taken place publicly and in accordance with Vietnamese law.
In March, Quynh received in absentia the International Women of Courage Award at the U.S. State Department, presented by first lady Melania Trump. Quynh was jailed at the time.
According to Human Rights Watch, there are more than 100 known political prisoners in Vietnam.
A court in central Vietnam on Monday sentenced activist Nguyen Van Hoa to seven years in prison after finding him guilty of producing online videos and interviews related to the fish kill that instigated anti-government protests. His conviction of spreading anti-state propaganda came after a half-day trial.
Vietnam denies it holds any political prisoners, saying only those who break the law are punished.
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