American defense officials raised the security alert level at military bases in Europe over the weekend in response to vague threats from the Kremlin about Ukraine using long-range weapons on Russian soil, U.S. and Western officials said.
Officials said no specific information had been gathered about possible Russian attacks on U.S. bases. Any such attack by Russia, overt or covert, would represent a significant escalation of its war in Ukraine.
Russia has stepped up sabotage in Europe, hoping to disrupt the flow of material to Ukraine. So far, no U.S. bases have been targeted in those attacks, but U.S. officials have said raising the alert level would help ensure service members remain vigilant.
Throughout the war, U.S. officials have assessed that President Vladimir V. Putin is reluctant to extend the conflict beyond Ukraine's borders.
But the surge in aid from the United States and Europe, and the loosening of restrictions on how the material is used, has caused consternation in Moscow, according to American officials. Recent Russian statements have made some American and European officials wary.
Ukraine has used American long-range missiles known as ATACMS to strike deep into occupied Crimea. The United States has also said Ukraine can use them in cross-border strikes against Russian military targets.
The Crimean attacks prompted Russia to summon Lynne M. Tracy, the U.S. ambassador, to the Foreign Ministry. And on June 24, a Kremlin spokesman said that any direct U.S. involvement in the war that killed Russians “must have consequences.”
The US decision to supply long-range weapons and ease restrictions on their use followed Britain's decision to supply Storm Shadow cruise missiles to Ukraine. Kiev had used those weapons to strike military targets in Crimea.
Attacks with Western weapons, particularly in Crimea, have proven effective, damaging Russian military logistics centers and further weakening Russia's Black Sea Fleet.
But the success of the attacks has prompted Moscow to look for ways to deter further attacks.
In recent months, Russia has stepped up a series of sabotage attacks across Europe. The campaign, led by Russian military intelligence, has at times appeared clumsy, including a fire at an Ikea store. But NATO has repeatedly warned of the incidents, and Britain expelled Russia's defense attaché after a fire at a London warehouse.
Military bases, which provide training, intelligence and other support to Ukraine, could be a logical next target, although there is no specific information indicating that Russia is considering such a strike.
Safeguarding military bases and the people who live and work there is part of what the Pentagon typically calls force protection. Beyond things like simple fences or guards protecting base gates, it consists of a series of increasingly restrictive security measures that can be implemented in proportion to a given threat.
Most U.S. military facilities around the world are at the second-lowest level of this type, called force protection condition “alpha,” which includes measures such as requiring officers to test their communications equipment and increased random checks of vehicles and personnel entering bases.
At the other end of the spectrum is the “delta” condition, set when an attack is imminent or in progress. That level shuts down non-essential functions like primary schools, directs all vehicles to be searched at entry gates, adds more guards, and severely restricts the movement of nearly everyone on a given basis.
Currently, U.S. military bases in Europe are in “Charlie” readiness status, the second highest level and the highest level of readiness that can be reasonably maintained for an extended period of time.
Over the weekend, Commander Daniel Day, a spokesman for U.S. European Command, said the military was asking personnel to “remain alert and vigilant at all times.”
In a statement released Monday, European Command said officials would not describe the measures it has taken to protect its operational security.
“Our increased vigilance is not related to a single threat,” the command said in the statement, “but rather an excess of caution due to a combination of factors that potentially impact the safety and security of U.S. service members in the European theater.”
Eric Schmitt contributed to the writing of the report.