Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

MOSCOW — A Russian official has indicated that American businessman Paul Whelan, who has been arrested in Moscow for suspected espionage, will be charged soon.

Also on Saturday, the country's Foreign Ministry said the U.S. had detained a Russian citizen, Dmitrii Makarenko, in the Northern Mariana Islands in the Western Pacific on Dec. 29. U.S. authorities had not notified the Russian Embassy of the detention, violating of international consular conventions, the statement added.

Whelan, a security executive and former Marine, was arrested on Monday by Russian officials.

Deputy Foreign Minister Sergei Ryabkov told the Russian news agency Interfax on Saturday, “[He] will be charged soon, as we understand."

While Ryabkov did not specify the exact charges Whelan might face, he added that the “question is very serious, it concerns intelligence activities in violation of the requirements of Russian legislation.”

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

The detentions on both sides look set to further complicate strained Moscow-Washington ties in spite of the stated desire of presidents Donald Trump and Vladimir Putin to improve relations between the countries.

Secretary of State Mike Pompeo said earlier this week that Washington had asked Moscow to explain Whelan's arrest and would demand his immediate return if the detention was deemed inappropriate.

Whelan was born in Canada and also has British and Irish citizenship. Officials from all three countries have said they are working to provide Whelan consular assistance in addition to the support being led by the U.S.

The former Marine served from 1994 to 2008, when he was discharged for bad conduct after being convicted on several charges of larceny, according to a Marine Corps spokesman.

Whelan's twin brother, Dave, has told NBC News he was visiting Moscow for a friend's wedding when he was detained.

NBC News reached out to U.S. officials for comment on Makarenko's arrest but received no reply.

Ryabkov dismissed the possibility of seeking an exchange of Whelan for Russian citizens. “I don’t see any reason to put the issue on the sphere of any exchanges. We have to go through all the procedures that are necessary in this situation,” he told Interfax.

In a separate interview with Russian state news agency TASS, Ryabkov warned citizens traveling abroad to exercise caution.

"We continue seeing a chain of episodes, incidents in which Russian nationals are detained in third countries at U.S. requests, and then they are extradited to the U.S. even if there is no basis for such actions. That is why it is necessary to be cautious," he said.

Separately, Russian officials said Makarenko had been detained after arriving on Saipan, the largest of the Northern Mariana Islands, and demanded Washington clarify the reason for the arrest.

"... Makarenko, born in 1979, has arrived on Saipan Island with his wife, underage children and elderly parents. He was detained by FBI personnel at the airport right after his arrival," the ministry said.

Papers filed in U.S. District Court for the Southern District of Florida show Makarenko was indicted in June 2017 for conspiring to export "defense items" without a license and money laundering.

Elena Holodny reported from Moscow, Linda Givetash and Tony Brown from London.