The United States and key Asian allies Japan and South Korea on Saturday agreed to bolster efforts to counter any threat from nuclear-armed North Korea, officials said.

US Defense Secretary Leon Panetta met with his South Korean counterpart Kim Kwan Jin, and Japan Parliamentary Senior Vice Minister of Defence Shu Watanabe on the sidelines of a regional security conference in Singapore.

"The ministers reaffirmed that North Korea's provocative behaviour threatens all three countries and will be met with solidarity from all three countries," the three said in a joint statement after their meeting.

"They agreed to continue to reinforce trilateral policy coordination in order to deter North Korean provocations."

Pyongyang's sinking of the South Korean warship Cheonan, its shelling of South Korea's Yeonpyeong island in 2010 and a missile launch in April "pose a serious threat to the peace and stability of the Korean Peninsula, Northeast Asia, and the world," the statement said.

"North Korea needs to understand that it will achieve nothing by threats or by provocations, and that such behaviour will only deepen its international isolation," it added.

The three defence officials are attending the weekend conference in Singapore called the Shangri-La Dialogue that ends on Sunday.

Before the meeting of the three ministers, the US defence chief said Washington would retain its troop presence at full strength in South Korea, despite budget pressures at home.

"We are also boosting our intelligence and information sharing with the Republic of Korea -- standing firm against hostile provocations from North Korea while transforming the alliance with new capabilities to meet global challenges," Panetta said in a speech at the conference.

Some 28,500 US troops are deployed in the South.