A Russian court on Thursday sentenced US basketball star Brittney Griner to nine years in prison over drug smuggling as US President Joe Biden called the ruling "unacceptable".

The court "found the defendant guilty" of smuggling and possessing "a significant amount of narcotics", judge Anna Sotnikova told a court in the town of Khimki just outside Moscow.

Sotnikova sentenced Griner, one of the world's greatest basketball players, to nine years in a Russian penal colony and said she would also have to pay a fine of one million rubles ($16,590).

"I love my family," Griner told journalists after the ruling as the six-foot-nine (2.06 metres) athlete was led away by members of Russia's law enforcement.

Her defence team told reporters the 31-year-old -- known as BG to her friends and fans -- was "very upset" and "very stressed".

Biden immediately released a statement, calling the Russian court's sentencing of Griner "unacceptable".

"Russia is wrongfully detaining Brittney," Biden said in the statement.

"It's unacceptable and I call on Russia to release her immediately so she can be with her wife, loved ones, friends and teammates."

The US president reiterated that he would "work tirelessly and pursue every possible avenue" to repatriate Griner.

Griner's trial accelerated in recent days as the United States and Russia discuss a potential prisoner swap that could involve the basketball star.

The White House urged Russia later on Thursday to accept its offer of a deal for the release of Griner and former US Marine Paul Whelan, who was imprisoned on espionage charges.

"We have made a substantial offer to bring her and Paul Whelan home. We urge Russia to accept that proposal," said White House spokeswoman Karine Jean-Pierre.

- 'Honest mistake' -

Griner was detained at a Moscow airport in February after she was found carrying vape cartridges with cannabis oil in her luggage.

The arrest came just days before Moscow launched its military intervention in Ukraine.

Prosecutors had earlier requested the two-time Olympic basketball gold medallist and Women's NBA champion be sentenced to nine and a half years in prison on drug smuggling charges.

Griner's trial came with tensions soaring between Moscow and Washington over Russia's military intervention in Ukraine that has sparked international condemnation and a litany of Western sanctions.

"I made an honest mistake and I hope that your ruling doesn't end my life here," Griner said earlier on Thursday.

"I want the court to understand it was an honest mistake that I made while rushing, under stress, trying to recover from post-Covid and just trying to get back to my team."

Griner pleaded guilty to the charges, but said she did not intend to break the law or use the banned substance in Russia.

Her defence team said they were "very disappointed" by the verdict and would appeal.

"The court completely ignored all the evidence of the defence, and most importantly, the guilty plea," lawyers Maria Blagovolina and Alexander Boykov said in a statement.

"The verdict is absolutely unreasonable. We will certainly file an appeal."

- 'Bringing BG home' -

Boykov told reporters the ruling was too harsh and many Russians usually get a suspended sentence for such a crime.

National Basketball Association and Women's National Basketball Association leaders said the athlete's sentence was "unjustified and unfortunate, but not unexpected".

"The WNBA and NBA's commitment to her safe return has not wavered and it is our hope that we are near the end of this process of finally bringing BG home to the United States," WNBA commissioner Cathy Engelbert and NBA commissioner Adam Silver said in a joint statement.

Her WNBA team the Phoenix Mercury said it was "heartbroken" for Griner but remained "confident in the public servants working every day to return her to her family and us".

"While we know it was never the legal process that was going to bring our friend home, today's verdict is a sobering milestone in the 168-day nightmare being endured by our sister, BG," the Mercury said in a statement.

Prosecutor Nikolay Vlasenko had said Griner "deliberately" proceeded through the green corridor at customs and stated she had nothing to declare "in order to conceal" the substance.

Standing inside a cage for defendants before the start of the hearing, Griner held up a photo of herself with teammates from the Russian club she plays for.

Griner was detained when she came to Russia to play club basketball with UMMC Ekaterinburg during the US off-season -- a common path for American stars seeking additional income.

The WNBA star said she had permission from a US doctor to use medicinal cannabis to relieve pain from her many injuries.

- Another swap? -

US Secretary of State Antony Blinken said last week Washington had made a "substantial proposal" to Moscow to free Griner and Whelan.

A prisoner swap was discussed during a call between Blinken and Russian Foreign Minister Sergei Lavrov on Friday.

The highest-profile Russian prisoner in the United States is Victor Bout, a 55-year-old arms trafficker, dubbed the "Merchant of Death", who is serving 25 years in jail.

There is no official confirmation that Washington has offered to exchange him.

Russia and the United States have already conducted one prisoner swap since the start of Moscow's Ukraine offensive.

In April, Washington exchanged former US Marine Trevor Reed for convicted drug smuggler Konstantin Yaroshenko.