A Vietnamese court has been criticised by the US after it found 14 pro-democracy activists guilty of subversion and sentenced them to jail terms ranging from three to 13 years.

The long prison terms suggest the country's Communist government is intent on stepping up its crackdown on dissenters to its authoritarian, one-party rule – particularly online.

The defendants are linked to Viet Tan, a Vietnamese dissident group based in the US. Vietnam has labelled it a terrorist group, but the US government says it has seen no evidence it advocates violence.

The court in central Nghe An province sentenced three defendants to 13 years during the two-day trial, which ended on Wednesday, according to defence lawyer Nguyen Thi Hue. She said 11 others received jail terms ranging from three to eight years. One of the three-year terms was suspended. The defendants, who include 12 Catholics, were arrested in late 2011.

Another defence lawyer, Tran Thu Nam, said they were found guilty of attending Viet Tan's overseas training courses on nonviolent struggle and computer and online security. Some also protested against China's territorial claims in the disputed South China sea, a sensitive issue for Vietnam because of the nationalist passion the issue provokes and Hanoi's ideological ties with Beijing.

The US wants closer ties with Vietnam because it sees it as a foil against China's power, but Hanoi's human rights record is a barrier. In December, human rights lawyer and blogger Le Quoc Quan was arrested. Last year, more than a dozen activists were sentenced to lengthy jail terms.

The US embassy said Wednesday's verdicts were "part of a disturbing human rights trend in Vietnam".

"We call on the government to release these individuals and all other prisoners of conscience immediately," it said in a statement.

Viet Tan said citizen journalists had been restricted by police to their hotel rooms during the trial.

"These activists have tirelessly advocated for social justice, engaged in citizen journalism and participated in peaceful demonstrations against Chinese territorial encroachment," it said in a statement. "The Hanoi regime has shown once again its fear of civil society."