South Korean President Moon Jae-in speaks at the Eastern Economic Forum in Vladivostok, Russia, on Sept. 7, 2017. (Sergei Bobylev/AP)

South Korea's new, liberal president took office this year with a pledge to foster warmer ties with North Korea. But the North Korean nuclear threat has rapidly escalated in the months since his election, leaving Moon Jae-in "sandwiched" between his supporters' expectations and the realities of dealing with an increasingly volatile regime, analysts say.

Moon was elected in May with broad support in a special election that followed the impeachment of his predecessor, Park Geun-hye. In electing Moon, who was wary of the United States and advocated dialogue with North Korea, voters eschewed nearly a decade of conservative governance in South Korea and Seoul's hard-line approach toward its northern neighbor.

Throughout his campaign, Moon vowed to take a two-track policy of pursuing dialogue with North Korea while maintaining pressure and sanctions to change North Korean behavior.

But North Korea has stepped up its nuclear and missile programs significantly since Moon became president — most recently by conducting on Sunday its sixth and most powerful nuclear test. And Moon is taking an ever-harder line toward North Korea, which some in his base perceive as a shift from his campaign assurances.

Moon's progressive supporters are not thrilled with the government's more hawkish rhetoric, while some of his harshest conservative critics are either confused or pleasantly surprised, analysts and advisers say.

In response to Sunday's nuclear test, Moon this week urged the international community to expand economic sanctions by cutting off critical crude oil to the North.

Sanctions have done little to alter North Korean behavior. Pyongyang on Thursday defended its nuclear program in a Russian-language statement released during an economic forum in Vladivostok, Russia, Bloomberg News reported. "We will reply to U.S. barbarian sanctions and pressure with our powerful countermeasures," the North Korean statement said.

In Vladivostok, Moon and Japanese Prime Minister Shinzo Abe agreed to push for tougher U.N. sanctions, including cutting off oil supplies, and pressing China and Russia to support such measures. Chinese Foreign Minister Wang Yi said on Thursday that Beijing would support further "necessary measures" by the United Nations against North Korea, although he did not specify what those might be.

The Trump administration also is pushing for further economic and diplomatic pressure on North Korea, but Russian President Vladimir Putin is reluctant to strengthen sanctions, saying such a move might hurt North Korean hospital patients and other ordinary citizens.

The South Korean government has made it clear that pursuing a dialogue has become a secondary part of the two-track policy. At an annual security forum on Thursday, Vice Foreign Minister Lim Sung-nam said, "The Korean government will do its utmost in close coordination with the international community so that every possible punitive measure can be taken."

Moon said his pursuit of tougher economic pressures on North Korea is a means to achieve a diplomatic and peaceful resolution — his two-track policy.

"I can say for sure that there will be no war on the Korean Peninsula again," Moon said Thursday in Vladivostok, Bloomberg News reported. 

Supporters are giving him time because they understand he is in a "sandwiched" spot and recognize that national security is the top priority from South Korea's perspective, advisers said. They warned that there will be a breaking point, although it is not clear what that would be.

"I've talked to many of my fellow progressives. They are still supportive and waiting, despite frustration and disappointment" over Moon's rhetoric on increasing economic pressures on North Korea, said Kim Joon-hyung, who was a key foreign policy adviser to Moon's presidential campaign.

Kim pointed out that Moon took office under extraordinary circumstances, following Park's impeachment. "There were candlelight demonstrations, a historical movement, that granted power to Moon. So they think Moon should be different," he said. "So far, he is unexpectedly hard-line."

But broadly, public opinion is sympathetic toward the harsher rhetoric, Kim said. For example, domestic support for nuclear armament and an American antimissile defense system in South Korea continues to grow. According to Gallup Korea polls, public support for deployment of Terminal High Altitude Area Defense (THAAD) anti-missile systems grew from 50 percent to more than 70 percent over the past year. 

In many ways, Moon's emphasis on pressuring the North mirrors public sentiments outside his core of support, said Kang Won-taek, a polling expert and professor of political science at Seoul National University. While Moon's core base may feel differently, the public in general has shifted toward a more conservative view on pressuring North Korea, he said.

Some conservatives are cautiously supportive of Moon in his call for stronger U.N. sanctions. 

"Many conservatives are satisfied with the shifting direction, said Shin Beom-chul, an expert on North Korea and defense at the government-run Korea National Diplomatic Academy in Seoul. "But they sometimes criticize that President Moon should have adopted that [approach] earlier, even from the beginning, in May or June."

Conservatives, he said, "ask for more — stronger support on the alliance, stronger sanctions on North Korea."

But conservative party leaders continue to slam Moon as failing to rein in North Korea and are pushing for a redeployment of U.S. tactical nuclear weapons in South Korea, an option that South Korea's defense minister said was worth reviewing

The main opposition party leader, Hong Joon-pyo, chairman of the conservative Liberty Korea Party, who ran for president against Moon, criticized Moon as failing to deliver on his campaign pledge to take the "driver's seat" in leading a multilateral approach to Korean denuclearization.

"It's just faint motion, a gesture, in the face of a political crisis. My supporters and I don't believe he really believes in increasing pressure," Hong said.

"For 20 years, we have repeated dialogue, sanctions, dialogue, sanctions. Despite that, North Korea has consistently developed its nuclear power. Now, they're at the final stages, at a point where dialogue is meaningless," Hong said. "It has become extremely clear that diplomacy or dialogue is not a solution. That means the only path we can take is through operating tactical nuclear weapons."

Foreign policy advisers to Moon said that it would be politically possible for him to shift to emphasize dialogue again in the two-track policy approach — but that such a change would need to be framed as a long-term goal.

"What the public wants now is weakening the immediate threat from the North, so to do that, you need THAAD and a tougher stance," Kang said. "In the long run, you might need dialogue. For now, in public opinion, they're supportive of Moon's tougher stance. Even though South Koreans are accustomed to life under military threat of North Korea since the end of the Korean War, the impact of the recent nuclear test is enormous."