By Barbara Starr

CNN has learned that U.S. intelligence agencies have identified three Russian amphibious warships in the eastern Mediterranean that are believed to be carrying weapons shipments that might be used to resupply the Syrian regime, according to a Pentagon official.

The official declined to be named due to the sensitive nature of the information.

The United States has been tracking the ships since they left Russian ports several days ago. U.S. satellites were able to see some indications of containers being loaded onto the ships. Although it's not confirmed, it's believed the ships may be carrying some components of the controversial Russian S-300 air defense missile system and other weapons for the regime.

The United States has been pressing the Russians for weeks to not deliver that system to Syria because of the threat it would pose by upgrading Syria's already robust air defense system. The United States believes it would give the Syrian government a much greater ability to target U.S., Israeli, NATO or other aircraft that may try to strike targets on the ground in the future.

The official said the United States did not see any military helicopters being loaded; it is believed the Syrians want to add the helicopters to their inventories.

The Russians have kept a regular naval presence in the eastern Mediterranean so they can move in and out of the port of Tartous, where they have facilities.

Several sensitive military assets are in the region this month as well. In the next two weeks, the U.S. Navy aircraft carrier USS Eisenhower will also be in the region on a scheduled rotation as it returns to its home port on the East Coast. A U.S. Patriot missile battery and F-16s are heading to Jordan for a training exercise, though the Jordanians have asked for the Patriots to remain after the exercise concludes at the end of the month.