Vietnamese police arrested four activists for anti-state activities today, the police ministry said, in a widening crackdown on critics of the Communist Party that human rights groups have described as alarming.

Despite sweeping reforms to Vietnam's economy and growing openness to social change, the ruling party does not tolerate criticism.

The activists arrested were linked to Nguyen Van Dai, a lawyer who was arrested in 2015 for anti-state propaganda, the police website said. They were being investigated for "activities aimed at overthrowing the government."

Dai, who has not been tried since his arrest, would now be prosecuted on the same charge, police said.

The activists were named as Pham Van Troi, Nguyen Trung Ton, Truong Minh Duc and Nguyen Bac Truyen. They were not available for comment and it was not clear whether they had legal representation.

The arrests come days after the United Nations condemned the jailing of activist Tran Thi Nga for nine years on charges of spreading propaganda against the state.

Her sentence came less than a month after another prominent female blogger, Ngoc Nhu Quynh - known as Mother Mushroom - was jailed for 10 years.

The U.S. ambassador in Vietnam, Ted Osius, has expressed concern over what he described as "deeply troubling" arrests, convictions, and harsh sentences of peaceful activists.

Vietnamese authorities have been accused of using vague laws to stifle bloggers and activists, who are getting more exposure from the proliferation of the Internet and social media in Vietnam, which has one of Asia's highest concentrations of Web users.