South Korean President Park Geun-hye and U.S. counterpart Barack Obama agreed to work closely together to adopt strong U.N. sanctions against North Korea’s latest nuclear provocation in their telephone conversation Thursday morning, Cheong Wa Dae said.

The two leaders shared the gravity of Pyongyang’s fourth nuclear test and the need for stern countermeasures in the phone call that was made at 09:55 a.m. and lasted for about 20 minutes, the presidential office said.

Obama reportedly said the North’s nuclear test hampers the regional stability and violates the obligations stipulated in the U.N. Security Council resolution and the six-party talk agreements, and pledged the U.S. would take all necessary measures to ensure its ally’s safety.

Park thanked Obama for his commitment and said she hoped for close cooperation in the future.

The two sides also emphasized the need for the most powerful and comprehensive sanctions against the North and agreed on the importance of three-way cooperation with Japan, as well as with China.

Toward the end of their talks, Cheong Wa Dae said Obama also congratulated Park on reaching an agreement over the long-standing issue of Japan’s wartime sex slavery of Korean women, and praised Park’s courage and vision in yielding a righteous outcome.

He also reportedly said the agreement on sex slavery would strengthen the capacity of Korea, Japan and the U.S. in dealing with the common challenge of North Korea’s nuclear test.

A day before, the defense ministers of the two allies also held talks over the phone, with U.S. Secretary of Defense Ash Carter reaffirming Washington’s commitment to the defense of the peninsula including through its extended deterrence.