WASHINGTON — The Russian military has been accused of harassing a U.S.-flagged cargo ship carrying equipment for an annual military exercise in the Baltic region that wrapped up Friday.

At least one Russian naval vessel and helicopters harassed the M/V Green Ridge in late May as it was bound for a port in Lithuania, an encounter the ship's master characterized as "intense and threatening," per an internal report on the incident obtained by Defense News.

The incident is reminiscent of multiple recent encounters between U.S. and NATO forces and the Russian military in that region, which has become increasingly unstable since Russia's incursion into the Crimea in 2014. But experts warn that the harassment of unarmed U.S. shipping carrying military equipment highlights a potential vulnerability in the ability of the U.S. to move heavy equipment to the region.

In a statement, U.S. Transportation Command said it is aware of the incident and acknowledged the ship was carrying equipment for Exercise Saber Strike 2017, which involves U.S. and NATO troops.

"We are aware of an alleged situation on May 25, involving a contracted, U.S.-flagged vessel, and Russian military ship and aircraft," the statement read. "The commercial ship was bound for Lithuania, and was contracted by the U.S. Department of Defense to transport equipment for an upcoming planned exercise."

The U.S. operates regularly in the area and expects everyone to behave safely, the statement read.

"The U.S. does, on a normal and routine basis, operate ships and aircraft in the Baltic Sea consistent with international law. U.S. aircraft and ships routinely interact with other nations, INCLUDING RUSSIA, in international seas and waters, and most interactions are safe and professional."

"While under DoD contract, commercial shippers are expected to observe these same maritime laws. We expect Russian vessels and aircraft to operate safely and professionally as well."

The Green Ridge is a vehicles carrier listed at about 57,500 gross tonnage.

According to the internal memo on the incident, the encounter happened close to Russian waters.

"The captain of the Green Ridge ... reported it was harassed by Russian ships and helicopters nearing Russian waters on its approach to Klaipeda, Lithuania, anchorage," the memo said. "The captain characterized the harassment as intense and threatening. The vessel was followed to the Lithuanian border."

The report was unclear what actions the Russians took that were deemed threatening.

The exercise included about 11,000 U.S. and NATO troops, according to a Defense Department release, and was spread out over most of the Baltic and Poland. The participants included  Belgium, Canada, Croatia, Denmark, Estonia, Finland, France, Germany, Italy, Latvia, Lithuania, Luxembourg, Netherlands, Norway, Poland, Portugal, Slovenia, Slovakia, the United Kingdom and the U.S, the release said.

Emails seeking comment on the Green Ridge episode were not returned by the Russian embassy in Washington.

Brooke Grehan, the chief operating officer at Central Gulf Lines, the company that was operating the M/V Green Ridge at the time of the incident, also acknowledged the incident in an email.

"Our US Flag vessel, M/V Green Ridge, during transit to Lithuania, did have communications with a Russian naval vessel," Grehan wrote. "When transiting congested areas such as the Baltic Sea there is often interaction between vessels, including naval vessels. Normally the communication between vessels is professional. However there are occasions when it is not. Our crew responded professionally and there was no delay as a result of incident."

A US vulnerability?

The intercept of a U.S. flagged ship near Russian waters is a continuation of an increasing pattern of aggressive maneuvers around U.S. and NATO military units in recent years, said retired U.S. Adm. James Stavridis, a former supreme allied commander and head of the Fletcher School of Law and Diplomacy at Tufts University.

"If the reports are true of a U.S. flagged merchant ship, this would be of a piece of ongoing Russian harassment of aircraft and Navy ships that has been occurring on an increasing basis for the past three years," Stavridis said. "What’s new here is that it’s a U.S. flagged merchant vessel," he said, adding that the actions as reported were dangerous and unacceptable.

But as tensions rise in the region over Russian actions in Ukraine and in Syria, experts fear the intercept could be an ominous reminder that the U.S. commitment to the defense of Europe is heavily dependent on shipping, which could help explain why Russia tried to intimidate an unarmed merchant.

Russian harassment of the Green Ridge, for some experts, highlights what could be a critical weakness in the United States’ ability to project power in the Baltics.

"The U.S. couldn’t support armed forces forward anywhere in the world without some degree of reliance on private commercial shipping," said Dan Goure, a former George W. Bush-administration official and analyst with the Lexington Institute think tank in Virginia. "Russia may try and make it uncomfortable for those companies, and if the companies decide, ‘We’re not going to do this anymore; it’s too much trouble,’ the U.S. will have a problem on its hands."

"So it may just be intimidation but its intimidation with a purpose," Goure added.

The issue has relevance in the ongoing debate within the U.S. Navy on its new frigate, which has traditionally been used to perform anti-submarine and convoy escort duties, said Jerry Hendrix, a retired Naval Flight Officer and analyst with the Center for a New American Security.

"It’s a very aggressive signal to the whole alliance structure," Hendrix said. NATO fully understands how important heavy shipping is to its defense, and intimidating that structure lets the allies know that Russia could target it.

In a recent reporton the Greenland-Iceland-United Kingdom gap, a major submarine operating lane for Russia, Hendrix and fellow CNAS analyst Julianne Smith highlighted the frigate shortfall, as well as the slipping attack submarine numbers, as a threat to convoys that would be crucial to supporting a war in Europe.

"The Atlantic-facing members of NATO now possess far fewer frigates — the premier class of surface vessels designated to conduct ASW operations — than they did 20 years ago," the report reads. "Where they collectively had around 100 frigates in 1995, that number hovers at 51 today. Similarly, these nations had, in 1995, 145 attack submarines — those dedicated to anti-shipping and anti-submarine warfare missions — but that number has plummeted to a present low of 84.

"As a result, NATO’s ability to monitor and track threats in the underwater environment has been badly degraded, just as a revanchist Russia is re-emerging to challenge NATO interests in the Atlantic Ocean and the Mediterranean Sea," the report says.

The Navy retired its last Oliver Hazard Perry-class frigate in 2015, though the platform had long-since been defanged and used mostly for presence and counter-narcotics patrols. The Navy is today finalizing designs for a new frigate that with anti-submarine and anti-air capabilities and is expected to release a formal request for proposal to industry by the end of the fiscal year.

David B. Larter was the naval warfare reporter for Defense News.

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