WASHINGTON (AP) — U.S. and Lithuanian troops and authorities are struggling to drain a swampy area in Lithuania in an ongoing effort to recover four American Army soldiers who went missing three days ago. The soldiers' armored vehicle was discovered submerged in 15 feet of water and so far teams have been unable to get inside or pull it out.

U.S. Army Europe and Africa said Friday that cranes, a large-capacity pump and more than 30 tons of gravel have been brought in to help drain the water, dig out the mud and pull the M88 Hercules out of the swamp. They warn that this will be a long operation.

In a statement, the Army said that the area around the site is “incredibly wet and marshy and doesn’t support the weight of the equipment needed for the recovery of the 70-ton vehicle without significant engineering improvements. Draining the area has been slow and difficult due to ground water seepage.”

The four soldiers, whose identities have not been released, went missing early on Tuesday when they were on a mission to recover another Army vehicle during unit training. The Hercules was found on Wednesday, and teams have been working since then to get the vehicle out.

The soldiers are part of the 1st Brigade, 3rd Infantry Division. A U.S. Navy dive crew is being brought in, and the Polish Armed Forces have volunteered to send engineers, additional equipment and 150 personnel to also assist.

“This will be a long and difficult recovery operation, but we are absolutely committed to bringing our soldiers home," said Maj. Gen. Curtis Taylor, commander of 1st Armored Division. "I remain incredibly impressed by the discipline, commitment and comradery in this unit as they attempt to recover their missing comrades.”

The soldiers were reported missing during an exercise at the General Silvestras Žukauskas training ground in Pabradė, a town less than 6 miles (10 kilometers) from the border with Belarus.