WASHINGTON - The United States distanced itself on Wednesday from a visit by former US ambassador to the United Nations Bill Richardson to Russia where WNBA star Brittney Griner and former US Marine Paul Whelan remain jailed.

Mr Richardson, who has privately worked to secure the release of American detainees abroad, held meetings there this week, according to a source familiar with the matter.

The US State Department indicated that any outside effort to help secure the release of detained Americans abroad should be fully coordinated with it and that such attempts run the risk of complicating matters.

"In this case we believe that any efforts that fall outside of that officially designated channel have the potential to complicate what is already an extraordinarily complicated challenge that we face," department spokesperson Ned Price told a news briefing.

He said the US government has been in touch with the Richardson Centre but said Mr Richardson's travel was not coordinated with the US embassy in Moscow. The centre specialises in negotiating releases of prisoners and hostages.

The source familiar with the matter said Mr Richardson, a former governor of New Mexico, was on his way back but did not provide further details on whom he met with or how his meetings went.

Mr Richardson has been involved in the release of several high-level prisoners, including that of American Danny Fenster from a Myanmar prison last year.

The Kremlin on Wednesday had no comment on Mr Richardson's trip.

"There were no meetings at the Kremlin," spokesman Dmitry Peskov told reporters on a conference call. "I have nothing to tell you on this subject."

A spokesperson for Mr Richardson said he was unable to comment on this at the moment. The Richardson Centre said it could not comment.

Griner, a two-time Olympic gold medallist and Women's National Basketball Association (WNBA) star, was sentenced to nine years in a Russian prison on drug charges on Aug 4, a verdict that US President Joe Biden called "unacceptable".

The United States in August said it has put forward a "substantial offer" on the table to secure the release of the two Americans.

Russia's foreign ministry said last month it was engaged in "quiet diplomacy" with the United States about a potential prisoner swap that could include Griner and Whelan.

But there have been few public developments on the issue over the past weeks.

Washington has offered to exchange Russian arms trafficker Viktor Bout for Griner and Whelan, sources familiar with the situation have told Reuters.

Whelan, who holds American, British, Canadian and Irish passports, was sentenced in 2020 to 16 years in jail after being convicted of spying. He denied the charge.

Griner, who had been prescribed medical cannabis in the United States to relieve pain from chronic injuries, was arrested at Moscow's Sheremetyevo airport in February with vape cartridges containing hashish oil in her luggage.

Cannabis is illegal in Russia for both medicinal and recreational purposes. REUTERS