Russia said Saturday it wanted an explanation from Washington over the arrest of one of its nationals, as Moscow continued to hold a US citizen for alleged espionage.
FBI agents arrested Dmitry Makarenko on December 29 on Saipan, a US island in the western Pacific and he had since been taken to Florida, a Russian foreign ministry statement said.
It did not detail the accusations against him but said the US authorities had failed to inform them of his arrest and they had only found out from his family.
Meanwhile, a top Russian diplomat on Saturday 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 believe his arrest was in retaliation for last year's arrest in the US of a Russian woman called 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 citizenship.
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.
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