The U.S. Navy (search) has dispatched two vessels to Nias island (search) in the earthquake-struck region of Indonesia, a Pentagon spokesman said Wednesday.

The USNS Mercy (search), a hospital ship, and the USNS Niagara Falls (search), a supply ship, are under way and should arrive at Nias island in six days, Lt. Cmdr. Greg Hicks said. The two vessels assisted in the recovery efforts from the December tsunami, and have since been on a humanitarian mission to East Timor.

The ships were sent to Nias island in response to an informal request for assistance from Jakarta, Hicks said. A formal request for help is expected through diplomatic channels shortly.

Other U.S. military assets, including aircraft and stockpiles of medical supplies, are standing by at Pacific Ocean bases if additional requests for aid are transmitted, Hicks said.

Monday's 8.7-magnitude quake struck off Indonesia's Sumatra island, some 75 miles north of Nias. The larger Dec. 26 quake that generated devastating tsunami hit an area further northwest along the Sumatran coast.

Indonesian officials have estimated that 1,000 people died in the latest disaster, but some fear the number could climb to 2,000. Bodies were still being discovered in the ruins of houses and shops on Wednesday and laid out in front of churches and mosques.

The confirmed death toll was put at 518, with 500 of those on Nias, United Nations officials said.