Vietnam stepped up suppression of dissent last year, targeting activists and attempting to curb criticism in its growing blogosphere, the US State Department said Thursday.

Echoing recent statements by US officials, the State Department's annual report said the human rights situation "remained a problem" in Vietnam.

"During the year the government increased its suppression of dissent, arresting several political activists and convicting others arrested in 2008," it said.

"Despite the continued growth of Internet blogs, the party and the government increased efforts to suppress press freedom," it said.

The State Department said Vietnam was holding at least 60 political prisoners at the end of 2009 including lawyers, activists and bloggers but acknowledged some observers put the number in the hundreds.

However, the State Department said that unlike in previous years, there were no reports of deaths in custody. Separately last week, Vietnam freed leading dissident Le Thi Cong Nhan, a lawyer.

The State Department report also said Vietnam was becoming less strict in its enforcement of restrictions against religion and pointed to the government's efforts to reconcile with the Vatican.

But it said the government still curbed freedoms for unapproved Buddhist and Christian groups.

The report pointed to what it described as vigilante attacks on followers of Thich Nhat Hanh, one of the world's most influential Buddhist monks.

"The attack occurred in the presence of uniformed and plainclothes police, who did nothing to prevent it," it said.

US senators, including Democrat Barbara Boxer and Republican Sam Brownback, are urging President Barack Obama's administration to redesignate Vietnam a "country of particular concern" on religious freedom, which would bring diplomatic pressure and potentially sanctions.

"Vietnam is backsliding on human rights and religious freedom and it's time for the Obama administration to take meaningful action," said Leonard Leo, chair of the US Commission on International Religious Freedom, an advisory body.

"US policy must champion both increased trade and guaranteed rights and should be clearly signaling that these issues cannot proceed separately," he said.