US bases in Europe placed on heightened security alert due to 'combination of factors': Pentagon

US commanders in Europe have placed bases in a higher state of alert due to a "combination of factors," the Pentagon said Monday.

"I won't speak to intelligence matters…but it was due to a combination of factors potentially impacting the safety and security of service members stationed in the European theater," spokeswoman Sabrina Singh told reporters.

"US European Command is taking steps to increase vigilance for our service members, their families and our facilities," she added.

Stressing that it was at the commanders' discretion, Singh said, without getting into more specifics: "This was done out of an abundance of caution."

Asked whether raising security threat levels correlated with the UEFA Euro Cup and the Olympic Games, she said: "You mentioned some very large, big events that are taking place in Europe. That's certainly a factor, but it's one of many factors."