The 53-year-old has been behind bars since 2018 and is serving a 16-year sentence that the US government says is without merit.

On Tuesday afternoon he was "hit in the face" by a new prisoner, breaking his glasses, his brother David Whelan said in a statement.

He said the incident occurred in a clothing workshop in the Mordovia penal colony in central Russia.

Guards do not enter that part of the prison and other inmates eventually came to Whelan's aid, his brother said.

"Paul is a target because he is an American and anti-American sentiment is not uncommon among the other prisoners," he added.

"Paul says he believes the prison administration is taking the attack seriously."

The US embassy in Moscow, which has been in contact with Whelan by phone, learned that he is receiving medical treatment following the incident, a State Department spokesperson said.

The State Department urged the Russian government to keep him safe and called for his release.

Whelan worked in security for a US vehicle parts company when he was arrested in Moscow in 2018 and has always asserted that the evidence against him was falsified.

Russia and the United States each accuse the other of detaining each other's nationals for political purposes.

A Moscow court on Tuesday said it had extended until January the detention of US reporter Evan Gershkovich, who was arrested in Russia earlier this year on spying charges.

A series of prisoner exchanges have been arranged in recent years.