Beijing on Friday denied seeking to retaliate against relatives of a US-based journalist from the mostly-Muslim Uighur minority, one day after Washington voiced alarm about the case.

US-funded Radio Free Asia (RFA) said that since 2009, Chinese authorities have targeted one of its ethnic Uighur reporters, Shohret Hoshur, who left China in 1994 after authorities deemed two of his articles to be "separatist".

His work for the broadcaster has focused on the minority's far-western Chinese homeland Xinjiang, where violence has intensified since early 2013 in a series of bloody clashes and attacks.

RFA said, in a statement posted online this week by the New York-based Committee to Protect Journalists, that Hoshur believes three of his brothers "have been wrongly arrested as a means to intimidate and even silence him as a journalist reporting on sensitive issues in China".

One brother was sentenced last year to five years in prison for "violating state security laws", and the two others were being detained for "leaking state secrets" after discussing the trial by phone with Hoshur, according to RFA.

Hoshur "has even received phone calls from his family members asking him to leave his job at RFA, which he believes were made under pressure by authorities", the broadcaster said.

US State Department spokeswoman Jen Psaki on Thursday said that Washington was "deeply concerned" about the case.

"We urge Chinese authorities to cease harassment of his family and to treat them fairly and with dignity," Psaki said.

"We continually urge China to respect internationally recognised human rights and fundamental freedoms, including freedom of expression," she added.

Asked Friday about the case, Hong Lei, a Chinese foreign ministry spokesman, dismissed the allegations as groundless.

"The reports you mentioned are totally baseless and are not worth refuting," Hong said at a regular briefing.

Beijing blames Xinjiang-related violence on "religious extremists", "separatists" and "terrorists", and has responded by launching a crackdown in the region, with hundreds of arrests and around 50 death sentences or executions announced since June.

The campaign has extended to academics such as prominent Uighur scholar Ilham Tohti, who in September was sentenced to life in prison for "separatism". Seven of Tohti's students have been jailed on the same charge.

Rights groups have condemned the targeting of Tohti, a respected economist and moderate who had long denounced the repression of Uighurs.

They argue that harsh police treatment of the minority as well as government campaigns against religious practices such as the wearing of veils have fomented violence.