South Korea and the United States condemned North Korea's claimed hydrogen bomb test on Wednesday as a "grave provocation" and vowed to strengthen cooperation in their response, the South Korean defense ministry said.

In a telephone conversation, South Korean Defense Minister Han Min-goo and his U.S. counterpart agreed that the North's fourth nuclear test is a "grave provocation that seriously threatens the peace and security not just on the Korean Peninsula but in the world," the ministry said.

North Korea claimed earlier that it has succeeded in conducting an H-bomb test, a provocative move that could prompt the U.N. Security Council to tighten its sanctions on the communist country.

South Korea's Ministry of National Defense said Han and Carter discussed ways to counter the North's actions, with Han scheduled to announce the details negotiated on Thursday.

"South Korea and the U.S. are discussing ways to respond to the North's latest nuclear test and prevent more provocations," the ministry said.

Earlier Wednesday, the head of South Korea's Joint Chiefs of Staff, Lee Sun-jin, and the commander of Combined Forces Command, Curtis M. Scaparrotti, talked over the phone to discussed ways to cooperate.

South Korea's top nuclear envoy also began full-fledged consultations with his American and Japanese counterparts on ways to deal with North Korea's claimed nuclear test.

Hwang Joon-kook, special representative for Korean Peninsula peace and security affairs, had phone talks with his U.S. counterpart Sung Kim, according to the Foreign Ministry.

They compared notes on the initial assessment of the current situation and discussed future steps through the U.N. Security Council, it said.

The 15-member council is set to hold an extraordinary session in New York to touch on the North's claim that it has "completely succeeded" in a hydrogen bomb test.

Although the North seems to have conducted its fourth-known nuclear test earlier Wednesday, South Korean military officials says the tremor of the artificial quake is too weak to be based on hydrogen.

Hwang also telephoned Japan's nuclear envoy Kimihiro Ishikane.

He requested Tokyo take an active role in punishing Pyongyang.

Japan is a member of the U.N. council, said the ministry.

Hwang then met with Russian Ambassador to South Korea Alexander Timonin, who was quoted as saying that Moscow is taking the situation seriously. (Yonhap)