Russia had reinforced its military capacity in the Mediterranean before invading Ukraine on February 24, military observers said.
Thibault Lavernhe, regional communication officer of the French army in the Mediterranean, said Russia has doubled, if not tripled, its military capacity in the area in terms of destroyers, frigates and submarines.
The Russian presence in the Mediterranean could be used to send military reinforcements and arms to the area, and also fire cruise missiles from warships to support military operations.
Lavernhe revealed that US forces, which had decreased their presence in the area in the past 10 years, are also working to change tactics in the Mediterranean.
"Ukraine has changed things. The Americans are back. This hasn't been the case since the Cold War," the officer said, adding that where there are American forces, the Russians are there too.
The Mediterranean is of strategic importance to the world economy, with 65 percent of EU energy supplies and 30 percent of global commerce, according to the French foreign ministry, passing through the sea with shores in Europe, the Middle East and Africa.
There are currently around 20 Russian warships in the sea, Lavernhe told AFP.
The buildup came in the wake of the war in Syria when Moscow began deploying warships in the port of Tartus, the Russian navy's sole repair and re-fueling base in the Mediterranean.
What is new now, Lavernhe said, is that Russian naval forces are now spreading westward - north of Crete, west of Greece in the Peloponnese, and in the northern Aegean Sea, near the Black Sea.
“Russian ships are positioned to monitor the activity of allied forces,” he stressed.
Meanwhile, NATO monitors all activity in the Mediterranean from planes like the glass-nosed Atlantique 2 on Crete's base of Souda.
The long-range patrol aircraft made by France's Dassault is equipped with radar, a 3,200-millimeter camera and systems capable of detecting magnetic fields and picking up nearby radar signals.
The plane's tactical coordinator, Laurent, explains that the aircraft will pick up all vessels encountered during its flight and determine their national affiliation.
"All vessels of over 12 meters must be registered and have an active tracking beacon," he says, discreetly closing a folder with the image of a Russian landing craft on one of the pages.
"If that is not the case, we hail them to establish whether they are smugglers or (other kinds of) illegal activity," he adds.
This information is then shared with the French general staff and NATO command.
Lieutenant Johann, the aircraft's chief officer, who asked that his surname not be used, said that the plane's range enables it to fly as far as the Black Sea, but that could potentially antagonize the Russians.
"We are not in a crisis situation in this area. The objective is simply to safeguard European security," he said.