Brittney Griner, a US basketball star detained in Russia, pleaded guilty on Thursday to drug-trafficking charges.
Speaking through an interpreter, the WNBA star said she had "no intention" of breaking any Russian law.
"I was in a rush packing. And the cartridges accidentally ended up in my bag," she said.
Griner was detained in February at Sheremetyevo Airport in Moscow after authorities reportedly found vape cartridges containing cannabis oil in her luggage. She faces up to 10 years in prison if convicted.
The Russian foreign ministry said Griner could appeal against any decision in the case or ask for clemency after a verdict has been reached.
The White House on Wednesday said President Joe Biden called Griner's wife to assure her that he was working to secure the Olympian's release. They spoke after Griner appealed directly to the president via a handwritten letter. The Rev Al Sharpton also called on the president to arrange a meeting with the detained Olympian.
The US State Department designated Griner as "wrongfully detained", handing her case to the special presidential envoy for hostage affairs. Russia says Griner's detention is not illegal, despite strained relations with Washington over Moscow's invasion of Ukraine.
US Secretary of State Antony Blinken said officials from the US embassy in Moscow attended the trial and gave Griner a letter from President Joe Biden.
Mr Blinken also called for the release of Paul Whelan and other "wrongfully detained" Americans. Mr Whelan was sentenced to 16 years in prison in 2020 on espionage charges that his family and the US claim are false.
Russian media outlets have speculated that Griner could be part of a prisoner swap in exchange for Russian arms trader Victor Bout, who is serving a 25-year prison sentence for conspiracy to murder US citizens and for providing aid to a terrorist organisation.
Deputy Foreign Minister Sergei Ryabkov said on Thursday it was difficult to exchange prisoners with the US and suggested Washington stop discussing Griner's fate.
“The persistence with which the US administration … describes those who were handed prison sentences for serious criminal articles and those who are awaiting the end of investigation and court verdicts as ‘wrongfully detained’ reflects Washington’s refusal to have a sober view of the outside world,” Mr Ryabkov said.
The Associated Press contributed to this report