(Reuters) - The United States "will do what is necessary" to see that the Philippines is able to resupply a ship on the Second Thomas Shoal that Manila uses to reinforce its claims to the atoll, White House national security adviser Jake Sullivan said on Friday.

Sullivan said the U.S. would prefer that the Philippines conducts the resupplies of the small crew on the warship Sierra Madre, which Manila beached in 1999 to reinforce maritime claims in the South China Sea contested by China.

Speaking to the Aspen Security Forum conference in Colorado, Sullivan said the United States has made clear to China that its mutual defense treaty with the Philippines applies to the Sierra Madre.

"The most important thing right now is to see de-escalation and to see the ability of the Philippines to do resupplies," he said.

(Reporting by Trevor Hunnicutt and Eric Beech; Editing by Doina Chiacu)