The U.S. Embassy in Tajikistan on Thursday voiced concern over the detention of a BBC radio correspondent in the ex-Soviet nation.

In a video statement, U.S. Ambassador Ken Gross urged the Tajik authorities to adhere to transparent judicial standards and investigate Urumboi Usmonov's alleged mistreatment by security forces.

"We believe journalists shouldn't endure physical abuse or imprisonment for what they write," Gross said.

The BBC said in Wednesday's statement that Usmonov's family members reported that he appeared to have been beaten up.

Tajik authorities said 50-year-old Usmonov, a citizen of Tajikistan, is suspected of being a member of Islamic movement Hizb-ut-Tahrir, which is illegal in the Central Asian republic. He has been in custody since Monday's arrest.

Tajik police said Usmonov joined the banned group in 2009 to carry out "propaganda and promoting this movement in social networks." The BBC said the charges are unfounded and that Usmonov has been denied access to legal representation.

The government of Tajikistan that borders Afghanistan to the north has kept a tight grip on the media and sought to quash Islamic groups outside its control.