A doctor in Vietnam has been sentenced to four years in jail for "anti-state propaganda", state media reported, as part of a fresh wave of convictions in the one-party state accused of waging a crackdown on critics. Bloggers, activists and lawyers are routinely jailed in communist Vietnam, where a hardline leadership in place since 2016 is accused of tightening its grip on dissidents. At least 24 activists were convicted last year, with another 28 arrested, according to Human Rights Watch, making 2017 one of the worst years for activists in the country. The latest to be jailed is Ho Van Hai, 54, a doctor who was arrested in November 2016 over a series of blog posts criticising the government. He was sentenced to four years in prison after a one-day trial in Ho Chi Minh City on Thursday, state-controlled VNExpress reported. "Of the 75 articles posted online and stored in Hai's computer, the authorities identified 36 articles that violated the regulation on management, provision and use of internet services and information on the internet," according to the VnExpress report. Hai is also accused of calling for public protests against Taiwanese steel giant Formosa, which dumped toxic waste into the ocean killing tonnes of fish along Vietnam's central coast in 2016. The disaster hit livelihoods hard and sparked rare nationwide protests. Hai is the fourth person jailed in Vietnam this week, after three men were convicted Wednesday also under Article 88 of the criminal code -- anti-state propaganda, which rights groups say is vaguely worded and used to curb dissent. The US embassy in Vietnam on Friday said it was concerned over the fresh convictions, saying it "supports the right of people to freely express their opinions". "We call on Vietnam to release these individuals and all other prisoners of conscience immediately, and to allow all individuals in Vietnam to peacefully express their political views without fear of retribution", the embassy said in a statement. At least eight dissidents have been jailed so far this year, after four Buddhist activists were convicted last month for flying the flag of the defeated southern regime loathed by the ruling communists.