"Now we got to concentrate on Paul Whelan, and I think we have to make an effort to get him out before the end of the year," Richardson told CNN on Thursday.
Earlier in the day, Richardson's spokesperson told Sputnik that the ambassador is hoping to meet with Russian officials in the coming weeks to resume talks on Whelan's release.
Richardson said he is optimistic that Whelan can be freed but underscored that the worsening relations between the United States and Russia have been affecting the negotiations.
On Thursday, Russia released US women's basketball player Brittney Griner from prison in exchange for the United States releasing Russian businessman Viktor Bout. The latter was serving a 25-year sentence in the United States for alleged arms dealing.
In June 2020, a Moscow court sentenced Whelan to 16 years in prison for espionage. Whelan has denied the charges but did not challenge the court’s verdict in the hope of being part of a potential prisoner exchange.