An exiled dissident detained for months in China has been slapped with charges of fraud as tensions in the country rise with weeks left to the day the pro-democracy movement in Beijing's Tiananmen Square was crushed by Chinese troops 20 years ago.

Zhou Yongjun, a student leader during the spring 1989 protests, was arrested by police at the end of September last year while crossing into the southern city Shenzhen from Hong Kong, a network monitoring human rights abuses in China said.

"Tensions rise every year leading up to the anniversary, but there has been more tension specifically this year," Wang Songlian, research director of Chinese Human Rights Defenders, told Kyodo News.

She added that the group had documented several incidents in the past months where dissidents connected to the Tiananmen incident are harassed or summoned by police.

While the group did not have details to show if the fraud charges against Zhou were credible, it believed his arrest was tied to his involvement during the democracy movement, she said.

Zhou, an undergraduate at the time of the massacre, had been living in exile in the United States for many years, family members in Sichuan said when contacted.

He was sent to a labor camp for three years the last time he visited the mainland in 1998 and was probably trying to return this time to visit his ailing parents, they said.

The U.S. State Department on Wednesday said it was "disturbed" by reports of the fraud charges against Zhou, who holds a U.S. green card, the equivalent of permanent residency.

Embassy spokeswoman Susan Stevenson in Beijing said the embassy had raised the matter of Zhou's arrest with the Chinese Foreign Ministry and called for proceedings against him to be both "transparent and consistent" with Chinese laws.

"We have the understanding that contrary to Chinese legal requirements, his family was not informed of his arrest," Stevenson said.

Other human rights groups have also in recent months reported of heightened tensions in the run-up to the Tiananmen crackdown anniversary June 4.

In a statement released in April, Amnesty International said a crackdown on human rights activists is "escalating" as the anniversary approaches.

The international Human Rights Watch group on Wednesday said "the Chinese government continues to victimize survivors, victims' families and others who challenge the official version of events," and called for the Chinese government to recognize that "20 years of denial and repression have only caused the wounds of Tiananmen to fester, not heal."

Some 30 individuals involved in the protests remain incarcerated in Chinese prisons for offenses they committed two decades ago, according to estimates released by nonprofit organization The Dui Hua Foundation earlier this week.