US Deputy Secretary of State Kurt Campbell speaks during a forum hosted by the Council on Foreign Relations in Washington on Monday. (The council's YouTube account)

US Deputy Secretary of State Kurt Campbell on Monday highlighted deterrence steps of last year's summit declaration between South Korea and the United States as tools to harness amid resurgent calls for the allies to craft stronger defense measures following last week's summit between Russia and North Korea.

After Russian President Vladimir Putin and North Korean leader Kim Jong-un signed a "comprehensive strategic partnership" treaty in a show of their deeper military ties in Pyongyang on Wednesday, calls have resurfaced for Seoul to consider a nuclear option.

But Campbell appeared to be putting the brakes on the calls, pointing to the Washington Declaration that South Korean President Yoon Suk Yeol and US President Joe Biden issued during their summit in April to enhance the credibility of America's deterrence commitment.

"I think that the mechanisms that we've put in place between the United States and South Korea ... you talked about the Washington Declaration and the strategic initiatives that have been launched to underscore the added signification of American extended deterrence, particularly in situations like Korea, I think it's given us what we need to work with now," he said during a forum hosted by the Council on Foreign Relations.

"We just have to be purposeful on following through with the specific steps in the declaration," he added.

Key deterrence measures in the Washington Declaration were the establishment of the bilateral Nuclear Consultative Group, a body to discuss strategic and nuclear planning issues, and a US pledge to enhance the "regular visibility" of strategic military assets to the Korean Peninsula.

Particularly, the NCG was a culmination of the allies' efforts to enhance the credibility of the US' "extended deterrence" commitment to using the full range of its military capabilities, including nuclear weapons, to defend its ally.

But the summit between Kim and Putin raised renewed questions over South Korea's security as their new treaty calls for one side to provide military assistance to the other "without delay" in the event of an armed invasion -- in a pledge that amounted to a revival of their Cold War-era military alliance.

During a webinar on Friday, Allison Hooker, former senior director for Asia at the National Security Council, warned that a deepening military relationship between Russia and the North could push South Korea to consider seeking its own nuclear program.

Campbell concurred with her view, noting that there are "limits" in the North Korea-Russia partnership, but they cannot be ignored.

"I do agree with Allison though (that) the steps that Russia and North Korea are taking are causing countries in the region to rethink all of their military and other steps. So what we've seen is substantial increases in military spending, different focus in South Korea, in Japan and elsewhere across the Indo-Pacific more generally," Campbell said.

"The United States is quite focused on underscoring the strength of our extended deterrence to the countries of Northeast Asia ... Japan and South Korea in particular."

Asked to comment on what North Korea could potentially get from the new treaty with Russia, Campbell raised the possibility of Moscow providing assistance linked to the North's nuclear and missile programs.

"We believe that there are discussions about what North Korea gets in exchange and they could be associated with its nuclear or long-range missile development plans, perhaps other things in energy and the like," he said. "I think we are steading in the aftermath of the agreement and watching carefully but we are concerned."

Touching on how China would feel about the growing ties between Russia and the North, Campbell said that China may be "anxious."

"I think it is fair to say that China is somewhat anxious about what's going on between Russia and North Korea," he said. "They've indicated so in some of our interactions and we can see some tension associated with those things."

He also pointed out China's concern that Pyongyang could undertake provocative acts that would have regional security implications.

"I think it would be fair to say that China is probably worried that North Korea will be somehow encouraged to take provocative steps that could lead to a crisis in Northeast Asia. So we are watching carefully," he said.

"You've seen some movements across the DMZ that have led to kind of a brief exchanges of fire we've seen," he added, referring to North Korean troops' recent incursions into the Demilitarized Zone separating the two Koreas. (Yonhap)