U.S. Secretary of State Marco Rubio, left, and South Korean Foreign Minister Cho Tae-yul in this combined image / Yonhap

South Korean Foreign Minister Cho Tae-yul held his first phone call on Thursday with new U.S. Secretary of State Marco Rubio and they agreed to maintain close cooperation on North Korea's nuclear issues, the foreign ministry here said.

The phone conversation took place after U.S. President Donald Trump was sworn in on Monday (U.S. time) and the top diplomats of the two nations discussed bilateral relations, North Korean nuclear issues as well as trilateral cooperation involving Japan, the foreign ministry said.

Welcoming Rubio's inauguration, Cho expressed his expectation to further develop the 70-year strong South Korea-U.S. alliance under the Trump administration.

Rubio stated that their alliance is the main pillar of peace and security on the Korean Peninsula, as well as the region, emphasizing its importance and reaffirming the U.S.' iron-clad defense commitment to South Korea, the ministry said in a statement in Korean.

Both ministers agreed to maintain close cooperation on North Korea's nuclear issues while further developing the trilateral cooperation involving Japan.

At Rubio's invitation of Cho to visit the U.S., the ministers plan to coordinate details to hold in-person talks in Washington at the earliest possible date, the ministry also said.

During Thursday's talks, Cho stressed the importance of close communication between the nations, including the need for a phone call between acting President Choi Sang-mok and Trump. Rubio responded that he would make every necessary effort regarding this.

In a separate readout, State Department spokesperson Tammy Bruce said that Cho and Rubio discussed advancing bilateral cooperation to address "common challenges" in the Indo-Pacific. She did not elaborate, but those challenges appear to include North Korean threats and China's growing assertiveness.

"Secretary Rubio stressed that the U.S.-ROK Alliance stands as the linchpin of peace, security, and prosperity on the Korean Peninsula and across the Indo-Pacific," Bruce said. ROK is short for South Korea's official name, the Republic of Korea.

The two sides also underscored the "critical" importance of furthering trilateral cooperation among South Korea, the U.S. and Japan, according to the spokesperson. (Yonhap)