The United States said on Monday it had sent two more warships into the waters near Yemen, heightening its military presence in the area as concerns grow over the country’s civil war.

The U.S. Navy sent an aircraft carrier and a guided missile cruiser from the Gulf into the Arabian Sea on Sunday. It said in a statement the purpose was to “ensure the vital shipping lanes in the region remain open and safe.”

The move brings to nine the number of U.S. warships dispatched near Yemen, where Iranian-backed Houthi rebels are battling forces loyal to the U.S.-backed president.

A Pentagon spokesman, speaking to Reuters, denied reports the ships were on a mission to intercept Iranian arms shipments to the Shiite Houthi rebels.

Washington has not directly intervened in the Yemen conflict so far but it supports Saudi-led airstrikes designed to stop the Houthi rebels’ advance.

One of these strikes hit a Scud missile base controlled by the Shiite rebels in the capital Sanaa on Monday. The