Seoul, Jan 7 (IANS): South Korean President Park Geun-hye and US counterpart Barack Obama have agreed to push for strong UN resolutions over North Korea's fourth nuclear test, Seoul officials said on Thursday.

Park and Obama spoke by phone for about 20 minutes, sharing views that North Korea should pay a corresponding price for its nuclear test while agreeing to maintain close cooperation in adopting strong resolutions in the UN rapidly, Xinhua news agency reported.

Obama stressed the need for the strongest and the most comprehensive sanctions against Pyongyang, according to the South Korean presidential office Cheong Wa Dae.

North Korea on Wednesday announced that it had successfully conducted its first test of a hydrogen bomb, the fourth nuclear test in total, which Pyongyang claimed was a perfect success.

Obama vowed to take all necessary measures to defend the safety of South Korea, saying that its commitment to the defence of its ally is unflappable.

In response, Park appreciated Obama reaffirming Washington's defence commitment and expressing its strong will to tackle Pyongyang's nuclear test, saying that she anticipates close cooperation with the US at the UN Security Council.

Park and Obama agreed to address North Korea's nuclear issue with the most urgency and robust will, sharing views that North Korea's nuclear test should be tackled in cooperation with Japan and China.

Seoul's foreign ministry said earlier Thursday that South Korean Foreign Minister Yun Byung-Se and his US counterpart John Kerry had a phone conversation overnight to discuss countermeasures following North Korea's nuclear test.