The United States on Thursday urged China to press North Korea's new leader to exercise restraint, saying that South Korea would face "enormous pressure" to respond to any provocations.

Kurt Campbell, the top US diplomat on Asia, admitted that little was known about North Korea's young supremo Kim Jong-Un and warned that "provocative steps have the risk of triggering deeply unforeseen consequences."

"We need to handle the situation with the greatest care and we expect China in their deliberations with North Korea to ensure that that message is deeply understood," Campbell said at the Stimson Center think-tank, echoing remarks made on a tour of East Asia earlier this month.

North Korea in 2010 shelled an island in the South and was accused of torpedoing a warship, incidents that killed 50 people and which some analysts saw as a way for young heir Kim to prove his mettle.

Campbell, an assistant secretary of state, said that South Korean President Lee Myung-Bak, a close US ally, showed "remarkable restraint" after the deaths.

"But their leaders have made clear that they've reached a point that if they faced further provocations, they would have enormous pressure to respond. And we understand that," Campbell said.

China is the closest ally of isolated North Korea, although Campbell said that even officials in Beijing were in the dark about Kim Jong-Il's December 17 death until North Korean state television announced the news two days later.

Kim's death threw into flux US plans for fresh diplomacy with North Korea, including a possible resumption of American food assistance to the impoverished state and more formal talks on ending Pyongyang's nuclear program.

"We have made clear through both public channels and privately that we are prepared to start a new chapter to deal clearly with outstanding issues of nuclear matters and the like," Campbell said.

Campbell met Tuesday with senior officials from Japan and South Korea to coordinate action. In a statement, the countries urged North Korea to recommit to past agreements to end its nuclear program.