STOCKHOLM — Holiday makers in Sweden heading out Tuesday to enjoy summer weather on Gotland, a scenic island in the Baltic Sea, were jolted when armored personnel carriers and other military vehicles boarded their tourist ferry, which was then escorted by Swedish fighter jets and a warship.
In addition to being a tourist destination, Gotland is also a strategically important site, often referred to as Sweden’s “fixed aircraft carrier.” The Swedish military deployed four naval warships and an unspecified number of ground forces and warplanes in response to a major Russian naval exercise that has set off alarms regionally.
A United States Air Force C-130 landed briefly in Visby, Gotland, on Saturday, said Therese Fagerstedt, a press officer at the Swedish Armed Forces Headquarters. But she denied the flight had any connection to the Swedish military activities.
The result, Jan Thörnqvist, chief of joint operations with the Swedish Armed Forces, said in a statement, is “extensive military activity in the Baltic Sea, conducted by Russian as well as Western players, on a scale the likes of which have not been seen since the Cold War.”
Those ships, the Korolyov, the Kaliningrad and the Minsk, set sail on Tuesday from the Kaliningrad region, a Russian enclave on the Baltic wedged between Poland and Lithuania, accompanied by two minesweepers and a corvette, or small warship.
The flotilla plans to sail up the coastlines of Latvia, Lithuania and Estonia, former Soviet republics that are NATO members, and near the territory of Sweden and Finland, which are neutral countries. The marines will then stage a mock amphibious assault on Russian territory near St. Petersburg, the military said.
Pavel Felgenhauer, a military commentator for an independent Russian newspaper, Novaya Gazeta, said the exercise is part of operation Ocean Shield, a long-running effort to lift the Russian navy’s preparedness.
He said it did not appear to be related to current Russian military exercises near the border with Belarus involving 6,000 troops and 1,500 armored vehicles.
Another analyst said the Russian exercise was a response to increased NATO activity in Eastern Europe. Inevitably, the analyst, Ivan Konovalov, director of the Center for Studies of Strategic Trends in Moscow, said, “our activity rises, too.”
Tensions between Russia and the West ramped up sharply in 2014, after Russia annexed Crimea and intervened militarily in eastern Ukraine. The Kremlin staged major military exercises along its western borders, stirring long-dormant fears of Russian armored columns rolling across Europe.
In response, NATO reinvigorated plans to counter an old foe, strengthening ties to allied armies, increasing the number of troops and spies devoted to Russia and embracing the Pentagon’s revamped defense strategy that focuses more on potential threats from Russia and China and less on terrorism.
Over the past two years, the United States and its NATO allies have positioned about 4,500 soldiers in the three Baltic States and Poland, and have stationed several thousand other armored troops mostly in Eastern Europe.
In Brussels, allied defense ministers last year approved a plan to ensure that by 2020, at least 30,000 troops, plus additional attack planes and warships, would be positioned to respond to Russian aggression within 30 days.
The tensions are part of an expanding rivalry, and corresponding military buildup, between Washington and Moscow.
Two Air Force B-1B bombers flying a long-range training mission over the Black Sea in late May prompted Russian fighter jets to scramble and intercept the American warplanes. At least three times in the past two months, Russian fighter jets intercepted Navy P-8 surveillance planes over the Mediterranean.
And last week, Russian fighter jets intercepted three U.S. RC-135 reconnaissance planes over the Baltic and Black seas, the Russian Defense Ministry said. The Russian jets eventually flew away without incident.
NATO held its annual exercise in the Baltics in June, with Sweden and Finland participating, though they are not members of the alliance. Over 8,600 soldiers, 50 ships, a helicopter carrier and two submarines were involved. But it has not held exercises of that size since, a NATO spokesman said.
Western analysts suggested that Russia could be reacting more to the recent protests in Belarus, warning outside powers about involving themselves in the uprising. The Kremlin has long accused the United States of promoting revolutions in former Soviet republics, particularly Georgia and Ukraine.
“The Russians are showing that they’re willing to go there,” said Karlis Neretnieks, a retired, high-ranking officer and former president of the Swedish Defense University. “They are trying to scare off the West.”
Mr. Neretnieks said that President Vladimir V. Putin’s government, already dealing with protests in Siberia, is worried that the possible fall of Aleksandr G. Lukashenko in Belarus might spark further demonstrations in Russia. “They now want to divert domestic attention and create an outside threat.”
While the deployment of what the Russian military itself described as a battle-ready landing force was unsettling, Mr. Thörnqvist said in a statement published on the Swedish Armed Forces’ website on Monday that the risk of a military attack was still very low, though the conditions were unpredictable. “We are witnessing a deteriorated security situation in the vicinity of Sweden,” he said.
This was not the first time that Russian military activity had set off alarms in Sweden. In 2017 the government announced the reintroduction of compulsory military service “to resist an armed attack against Sweden from a qualified opponent,” a letter from the Swedish Civil Contingencies Agency said.
Stockholm was also worried about another threat at the time: the possible withdrawal of the United States security guarantee, as President Trump fulminated against NATO. “It has created high levels of concern all over Europe,” one analyst said.
Thomas Erdbrink reported from Stockholm and Andrew E. Kramer from Moscow. Reporting contributed by Eric Schmitt from Washington and Christina Anderson from Stockholm.