According to US defense officials, there are eight staging areas in Russia where large numbers of military forces appear to be preparing for incursions into Ukraine.

The situation escalated last week when Moscow accused Ukraine of sending sabotage teams armed with explosives into the Crimean peninsula, which Russia had formally annexed in 2014.

Kiev called the Russian allegations as “fantasies” fabricated to instigate a new border war. Despite Kiev’s protests, Russian Prime Minister Vladimir Putin further accused Ukraine of killing two Russians- a soldier and an FSB security officer- during the alleged sabotage missions. “This is a very dangerous game,” Putin warned on state television. “There is no doubt we will not let these things pass.”

It is estimated that as many as 40,000 Russian troops, including tanks, armored vehicles, and air force units, are now stationed along Ukraine’s eastern border with Russia. Additionally, Pentagon officers express concern over the planned military exercises the Russian army is planning on conducting on the border. These exercises could potentially be used as cover-ups for an actual attack into Ukraine.

“Russian units will likely practice reinforcing the [Crimean] peninsula through such activities as amphibious landings and air defense exercises, and this may involve the change out of equipment and long convoys of military vehicles,” one defense official said.

The military exercises are a legitimate cause for concern. Similar large-scale Russian exercises were conducted a month before Moscow carried out the covert military operation to take over the strategic Black Sea peninsula in March 2014.

Navy Capt. Danny Hernandez, a spokesman for the U.S. European Command, assures that the upcoming Russian exercises are being closely monitored. “We are extremely concerned about the increased tensions near the administrative boundary between Crimea and the rest of Ukraine,” he said. “We urge both sides to avoid provocative steps or rhetoric that could escalate the situation.”

As the tension increases, the West holds its breath, as if Putin’s choice of action may have a great impact on Russian’s relations with its Western allies.