The U.S. ambassador to Russia Lynne Tracy visited former U.S. marine Paul Whelan, who has been imprisoned in Russia on spy charges for over four years, on Thursday.
Permission to pay a visit to Whelan in Mordovia, a southeastern Russian region, comes just over two weeks after Tracy was given access to U.S. journalist Evan Gershkovich, who was arrested in March on espionage charges.
The prison visit also comes after Washington said "there was a proposal on the table for [the release of] Paul" in March.
Washington says both Whelan and Gershkovich are "wrongfully detained" and has accused Moscow of "hostage-taking."
The U.S. embassy in Russia said Tracy traveled to the IK17 prison in Mordovia — some 400 kilometers southeast of Moscow — to visit Whelan in a region notorious for its harsh prisons.
"The U.S. government will continue to engage Russian authorities on his case so Paul can come home as soon as possible," it said.
It called working on his release an "absolute priority."
National Security Council spokesman John Kirby said on CNN earlier Thursday that his government was "energetically trying" to get Whelan released.
"We will urge the Russians to accept that proposal so we can get him out of there," he said.
He said U.S. officials were also "in initial conversations ... trying to get the Russians to agree to release Evan."
He added the United States wanted "more consular access" to the journalist.
In March, US Secretary of State Antony Blinken said Washington had put forward a "serious proposal" that Moscow "should accept" for Whelan's release.
Whelan was arrested in 2018 and sentenced to prison two years later for alleged spying.
Both Russia and the United States accuse each other of making political arrests and have carried out several high-profile prisoner exchanges in recent years.
Last year the United States secured the release of basketball star Brittney Griner from a Russian jail, as well as that of another former Marine, Trevor Reed, who was imprisoned for assaulting a Russian police officer.