Russia demanded an explanation from the United States on Saturday over the arrest of one of its nationals, amid tensions between the two countries after Moscow held a US citizen for alleged espionage.
FBI agents arrested Dmitry Makarenko on December 29 on Saipan, the largest of the Northern Mariana Islands, a US commonwealth in the Western Pacific. A federal arrest warrant indicates he has since been taken to Florida.
While still a fugitive, Makarenko was indicted in June 2017 along with fellow Russian national Vladimir Nevidomy for money laundering and attempting to export defense-restricted goods -- military grade night-vision rifle scopes, monoculars and ammunition primers -- without obtaining the necessary licenses.
Nevidomy has since been tried and convicted. He was sentenced to 26 months in prison with three years of supervised released.
The Russian foreign ministry said US authorities had failed to provide information about Makarenko's arrest to Moscow, which only found out from his family.
Meanwhile, a top Russian diplomat said the case of Paul Whelan, the US national detained in Moscow, was very serious.
Whelan, a security official at a US auto parts company and former US Marine, was arrested on December 28 "while carrying out an act of espionage," the FSB security service announced.
"The situation around Mr Whelan is very serious," Deputy Foreign Minister Sergei Ryabkov told RIA Novosti news agency.
"He came to Russia, as we understand, to take measures to carry out intelligence activities in violation of Russian law," he said, indicating that Whelan had not yet been formally charged.
But Whelan's lawyer Vladimir Zherebenkov told RIA Novosti on Thursday that his client had been charged -- with espionage.
Whelan's family said he was visiting Moscow for a friend's wedding and US security experts have raised doubts that he was a spy, given a reportedly chequered history in the US military.
Some observers say his arrest came in retaliation for last year's arrest in the United States of a Russian woman named Maria Butina.
Butina was indicted and pleaded guilty to acting as an unregistered agent of the Russian government -- a legal charge sometimes used against foreign intelligence agents.
Analysts have speculated that Moscow might be hoping to swap Whelan for Butina or another Russian held by the United States.
Ryabkov said that given the fact that Whelan has not yet been charged, it was too early to talk about his possible release in a spy swap.
Although Whelan entered Russia on his US passport, he also holds British, Irish and Canadian citizenships.
Ryabkov said the question of which country's diplomats would have access to Whelan would be decided on a case-by-case basis, based on conventions on consular relations.