Defense Secretary Lloyd Austin has directed that a US Navy aircraft carrier strike group remain in the Mediterranean Sea region rather than moving on to the Middle East, citing concerns over a buildup of thousands of Russian troops near Ukraine’s border.

According to a defense official, the adjustment in the itinerary of the USS Harry S. Truman and the five American warships following her underlines the requirement for a constant presence in Europe. It is required to reassure US friends and partners in the region, according to the official, who spoke on the condition of anonymity to discuss military deployment details.

The United States and its Western allies have watched as Russia’s army buildup along the border increased to an estimated 100,000, raising worries that Moscow was poised to invade Ukraine.

Russia annexed Ukraine’s Crimean Peninsula in 2014 and quickly backed a separatist revolt in the country’s east. Over the course of more than seven years, the violence has killed over 14,000 people and ravaged Ukraine’s industrial heartland, known as the Donbas.

Russia has rejected any intention of launching a military invasion, accusing Ukraine of devising plans to use force to recover territory controlled by Moscow-backed rebels. Ukraine has denied this assertion.

The Truman strike group consists of five US ships: the cruiser USS San Jacinto, as well as the guided missile destroyers USS Cole, USS Bainbridge, USS Gravely, and USS Jason Dunham. The Royal Norwegian Navy frigate HNoMS Fridtjof Nansen is also on board.

The Truman sailed from its homeport of Norfolk, Virginia, on December 1 and arrived in the Mediterranean Sea on December 14. It was supposed to continue on towards the Gulf area.