WASHINGTON (AFP) - US President Barack Obama on Tuesday vowed a steadfast defence of South Korea in a telephone call with outgoing President Lee Myung-Bak after North Korea conducted an underground nuclear test.

"The two leaders condemned this highly provocative violation of North Korea's international obligations. They agreed to work closely together, including at the United Nations Security Council," a White House statement said.

The leaders, who have become friends over the past four years, pledged to pursue international measures to impede Pyongyang's nuclear and ballistic missile programmes and reduce the risk of proliferation.

"President Obama unequivocally reaffirmed that the United States remains steadfast in its defence commitments to the Republic of Korea, including the extended deterrence offered by the US nuclear umbrella," the statement said.

"The President also thanked President Lee for his leadership and friendship over the past four years and pledged to work closely with President-elect Park to further strengthen US-ROK cooperation."

Mr Park Geun-Hye, who takes office in Seoul on Feb 25, campaigned on a platform of greater engagement with Pyongyang.