An armed British ship carrying 730 pounds of weapons-grade plutonium left a port in Japan Tuesday, destined for a secret mission to South Carolina.

Authorities have declined to say anything about the trip, citing security concerns. But Kyodo News has reported that the shipment is part of a 2014 agreement Japan struck with the U.S. to decrease its massive stockpile of plutonium.

The ship, accompanied by a second vessel, is headed for the Savannah River Site, a nuclear reservation in South Carolina, where the plutonium is to be downgraded and stored. 

Both vessels, named Pacific Egret and Pacific Heron, are equipped with naval guns, the Associated Press reported, and departed from a port in Tokai village, a coastal town about 80 miles northeast of Tokyo. 

The ships first arrived in Japan Monday, taking several hours to load plutonium-filled casks. There was no word when they were to arrive in South Carolina, or even what port they were destined for. 

The plutonium handed over by Japan is no small amount, but still barely puts a dent into the country’s huge plutonium stockpile. Ostensibly for research into nuclear power, the island country owns nearly 50 metric tons, or enough for almost 6,000 atomic bombs.