Alert Level Raised at U.S. Bases in Europe Over Russian Threats

American defense officials raised the security alert level at military bases in Europe over the weekend in response to vague threats from the Kremlin over Ukraine’s use of long-range weapons on Russian territory, according to U.S. and Western officials.

Officials said that no specific intelligence about possible Russian attacks on American bases had been collected. Any such attack by Russia, whether overt or covert, would be a significant escalation of its war in Ukraine.

Russia has been stepping up acts of sabotage in Europe, hoping to disrupt the flow of matériel to Ukraine. So far, no American bases have been targeted in those attacks, but U.S. officials said raising the alert level would help ensure that service members were keeping watch.

Throughout the war, U.S. officials have assessed that President Vladimir V. Putin is loath to expand the war beyond Ukraine’s borders.

But stepped-up U.S. and European aid — and the easing of restrictions on how that matériel is used — has caused consternation in Moscow, according to American officials. Russia’s recent statements have made some American and European officials wary.

Ukraine has been using longer-range American missiles known as ATACMS to strike deep into occupied Crimea. The United States has also said Ukraine can use them in cross-border attacks on Russian military targets.

The strikes on Crimea prompted Russia to summon Lynne M. Tracy, the U.S. ambassador, to the Foreign Ministry. And on June 24, a Kremlin spokesman said any direct U.S. participation in the war that led to the deaths of Russians “must have consequences.”

The U.S. decision to both provide longer-range weaponry and loosen the restrictions on using it followed a decision by Britain to provide Storm Shadow cruise missiles to Ukraine. Kyiv had used those weapons to strike military targets in Crimea.

The strikes with the Western weaponry, particularly in Crimea, have proved effective, damaging Russian Army logistics centers and further weakening Russia’s Black Sea fleet.

But the success of the attacks has Moscow looking for ways to deter further strikes.

In recent months, Russia has stepped up a series of sabotage attacks at various places in Europe. The campaign, carried out by Russian military intelligence, has at times appeared ham-handed, including a fire at an Ikea store. But NATO has repeatedly warned about the episodes, and Britain expelled the Russian defense attaché after a fire at a warehouse in London.

Military bases, which provide training, intelligence and other support for Ukraine, could be a logical subsequent target, even if there is no specific intelligence that Russia is considering such an attack.

Safeguarding military bases and the people who live and work on them falls under what the Pentagon typically calls force protection. Beyond things like simple fences or guards protecting base gates, that consists of a series of increasingly restrictive security measures that can be carried out in proportion to a given threat.

Most U.S. military facilities around the world are at the second-lowest such setting, called force protection condition “alpha,” which includes measures like directing officials to test their communications equipment and increasing spot checks of vehicles and people entering bases.

At the opposite end of the spectrum is condition “delta,” set when an attack is imminent or underway. That level shuts down nonessential functions like base schools, directs the searching of all vehicles at entry gates, adds more guards and heavily restricts the movement of nearly everyone on a given base.

As of now, American military bases in Europe are at condition “charlie,” the second-highest level and the highest level of readiness that can be reasonably sustained over a long period of time.

Over the weekend, Cmdr. Daniel Day, a spokesman for U.S. European Command, said the military was asking personnel to “remain vigilant and stay alert at all times.”

In a statement on Monday, European Command said officials would not describe the measures they were taking to protect their operational security.

“Our increase in vigilance is not related to any one single threat,” the command said in the statement, “but out of an abundance of caution due to a combination of factors potentially impacting the safety and security of U.S. service members in the European theater.”

Eric Schmitt contributed reporting.