BEIJING (AP) -- The oldest brother of a Chinese activist who fled house arrest and later settled in the U.S. said he was beaten up Thursday by unidentified men, part of an increase in harassment possibly linked to the activist's plans to visit Taiwan.

Chen Guangfu said he was followed by a black car without a license plate while he was on a motor scooter visiting a relative in another village in Shandong province. The car overtook him and blocked his way before two men in their 20s got out and punched and kicked him, he said.

"It lasted three to five minutes," Chen said. "I shouted for help while struggling to get away."

Chen said his family's home has been pelted with rocks and its yard littered with dead poultry in recent weeks, and that posters have been placed around town labeling activist Chen Guangcheng and his relatives as traitors.

"The blind traitor is a running dog for America, where he leads a decadent life and gets paid for his sell-out," the poster reads. "Not only did he go to the U.S., now he wants to go to Taiwan!"

Chen Guangfu said he believes local government officials are behind the harassment.

Taiwan's government-owned Central News Agency has reported that Chen Guangcheng - who now lives in New York with his wife and children - plans to visit the island, likely for two weeks starting on June 23. It is unclear if Chen will meet with Taiwanese President Ma Ying-jeou.

Chinese authorities are not happy with Chen's high profile because he has come to embody China's brutal suppression of activists. They also frown on celebrity visits to Taiwan that might boost its image as a sovereign nation. Beijing considers the self-governing island to be part of its own territory.

Chen sparked a diplomatic crisis when he escaped brutal house arrest in his rural town in April 2012 and sought refuge in the U.S. Embassy in Beijing. The Chinese government later allowed him to move to the U.S. with his wife and children.

Since his escape, Chen has criticized China's human rights records, speaking about it before a U.S. congressional committee. He also has complained that the Chinese authorities have reneged on assurances made to the U.S. that his relatives would be treated according to law.

In Washington on Thursday, State Department spokesman Patrick Ventrell expressed deep U.S. concern about the latest reported harassment and urged Chinese authorities to stop it and treat Chen's family members fairly and with dignity. He said Secretary of State John Kerry recently wrote to senior Chinese officials conveying U.S. concerns.

Ventrell said he didn't have information on whether the harassment was linked to Chen's conduct in the U.S.

Chen's nephew, Chen Kegui, was sentenced to 39 months in prison after he clashed with local officials who stormed into his parents' house in the aftermath of the activist's escape, in what supporters said was retaliation for the embarrassment the case has caused Chinese officials.

The nephew is now suffering from appendicitis, and it is unclear if he's receiving any medical care, his father Chen Guangfu said.

Chen said tree and vegetable seedlings also have been pulled up by the roots from family fields. Another brother had all four of his car tires slashed when he visited their mother last month, Chen said.

Chen said he reported Thursday's assault to the local police.

A man who answered the phone at the local police station, who identified himself only by his family name, Gao, confirmed that police were looking into the incident.

"We sent people to the scene to investigate," he said. "But I don't know anything else."

Calls to the local township police went unanswered Thursday. A government official at the higher county-level office said he had no knowledge of the incidents before hanging up.

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Associated Press writer Matthew Pennington in Washington contributed to this report.