WASHINGTON (REUTERS) - The United States has made "a substantial offer" to Russia to release US citizens detained there, US Secretary of State Antony Blinken said on Wednesday (July 27), adding that he would be pressing his Russian counterpart to respond in a conversation planned for the coming days.

Washington offered Moscow a deal to bring home WNBA star Brittney Griner and former US Marine Paul Whelan weeks ago, Blinken said at a State Department news conference, and hoped to advance the process when he speaks to Russian foreign minister Sergei Lavrov.

Russia’s foreign ministry said it had not been formally approached by Washington to set up a call, Tass news agency said later.

"There was a substantial proposal on the table weeks ago to facilitate their release. Our governments have communicated repeatedly and directly on that proposal. And I'll use the conversation to follow up personally and I hope move us toward a resolution," Blinken said.

He declined to say what the United States was offering in return.  

A source familiar with the situation confirmed a CNN report that Washington was willing to exchange Russian arms trafficker Viktor Bout, who is serving a 25 year-prison sentence in the United States, as part of a deal.

A Russian lawyer for Whelan has previously said he believed Moscow wanted Bout to be part of a swap for Whelan.  

Bout’s wife Anna said she had learned about a possible exchange from media reports, RIA news agency said.  

“We talked on the phone yesterday. Viktor doesn’t know anything about talks between Russia and the United States about his exchange... neither he or I have any information,” it cited her as saying.  

Blinken declined to characterise how Moscow has reacted to the proposal, which he said had President Joe Biden’s sign-off. 

Families of hostages and detainees have been increasing pressure on Biden, most recently in the case of two-time Olympic medalist Griner, who has been held since February.

Griner, detained at Moscow’s Sheremetyevo airport with vape cartridges containing cannabis oil in her luggage, was in the courtroom on Wednesday in the latest hearing of her trial on drug charges. The next hearing is set for Aug 2.

“From a legal point of view, an exchange is only possible after a court verdict,” Griner’s lawyer in Russia, Maria Blagovolina, said in a statement.

Whelan was sentenced in 2020 to 16 years in prison in Russia, accused of spying. He denied spying and said he was set up in a sting operation. Washington demanded his release.

The plight of American detainees has gained visibility after Griner's arrest at a Moscow airport and the release in April of former US Marine Trevor Reed at a time when US relations with Moscow are at their worst in decades over Russia's invasion of Ukraine.

Reed was detained for three years. In a prisoner exchange, the United States commuted the prison sentence of Russian pilot Konstantin Yaroshenko, who was convicted of cocaine smuggling.

Blinken said his planned call with Lavrov, the first such conversation between the two diplomats since before the start of Russian invasion of Ukraine in Feb 24, will not be a negotiation about Ukraine.

The White House also declined to go into the details of the US proposal but said that Biden directed his national security team to pursue every avenue to bring Griner and Whelan home safely to their loved ones.

Whelan's family welcomed news of the US offer. "We hope the Russian government responds to the US government and accepts this or some other concession that enables Paul to come home to his family," David Whelan, Paul's brother, said in a statement.