US Secretary of State Rex Tillerson on Friday reaffirmed Washington's "ironclad" security alliance with South Korea amid growing military threats from a provocative North Korea.

The US top diplomat made the remarks during his meeting with South Korea's Acting President and Prime Minister Hwang Kyo-ahn, who took over in December after then President Park Geun-hye was impeached by parliament over a massive corruption scandal.

"I am here to express and reaffirm the ironclad alliance between the United States and the Republic of Korea which serves as a linchpin for security and stability on the Korean Peninsula," Tillerson said during the talks at Hwang's office in Seoul, the second leg of his three-nation trip that includes visits to Tokyo and Beijing.

"I also want to stress the importance of our strong people-to-people ties, growing economic cooperation and, as you mentioned, our strong alliance for security in response to the growing threat from North Korea," he added.

Hwang stressed that Tillerson's visit to Seoul underscores the importance of the close-knit partnership between the allies.

"This visit by the secretary of state shows the importance of the South Korea-US alliance and their close-knit partnership," the acting president said.

"Although (South Korea) is in a difficult transitional period of time, the secretary visited (Korea) as planned, and this is proof that South Korea and the US go shoulder to shoulder 100 percent," he added.

Hwang, in addition, highlighted that South Korea, "without wavering," has been striving to manage state affairs though the government is reeling from the fallout of Park's impeachment. 

"I hope that (Tillerson's) visit this time will serve as an opportunity to reinforce the bilateral determination again for cooperation in relation to the South Korea-US alliance and the combined defense posture," he said.

On Saturday, Tillerson is to depart for China, the last leg of his first trip to Asia since he took office last month. (Yonhap)