SEOUL (BLOOMBERG) - A few months ago, South Korean President Moon Jae In took office promising a new era of engagement with North Korea. Now he is pushing for a military overhaul to keep Kim Jong Un's regime at bay.

In the past few weeks, Mr Moon has sought stronger warheads on ballistic missiles, stepped up military drills, discussed the deployment of US strategic bombers to South Korea and embraced a missile defence system he had questioned.

He has also called on his generals to draw up a detailed timetable to complete a last-resort strategy to strike North Korean nuclear sites, intercept missiles and take out its leadership.

Mr Moon, whose May election win ended nine years of conservative rule, had little choice but to shift focus after Mr Kim tested two intercontinental ballistic missiles, fired a rocket over Japan and detonated North Korea's most powerful nuclear device yet.

Mr Kim has shunned overtures from Mr Moon, saying he would never negotiate away his nuclear weapons if the United States maintains its hostile policy.

"Moon is showing that he's pragmatic," said Mr John Blaxland, head of the Australian National University's Strategic and Defence Studies Centre in Canberra. "The circumstances are more dire than he faced when he took office. He isn't as doggedly ideologically left as some might have anticipated."

Still, for US President Donald Trump, Mr Moon has not gone far enough. Tensions between the leaders surfaced recently, when Mr Moon asserted a right to veto any US strike against North Korea.

After Mr Kim's regime detonated what it called a hydrogen bomb on Sunday (Sept 3), Mr Trump dismissed Mr Moon's approach as "appeasement". A day earlier, Mr Trump reportedly threatened to kill a US-South Korea free trade agreement.

While the two leaders spoke by phone on Monday and agreed "to maximise pressure on North Korea using all means at their disposal", tensions remain evident. Mr Moon's office rebutted a White House statement saying Mr Trump gave his approval for "many billions of dollars' worth of military weapons".

"There is little affinity between Moon and Trump, and grave concern in South Korea about where the Trump administration is taking things," Mr Blaxland said. "But Moon is messaging Trump to say that in South Korea you have a faithful partner."

Mr Moon has little choice but to rely on the US for now. South Korea hosts roughly 28,500 American troops and depends on the nation's nuclear protection to deter a North Korean attack.

Mr Trump's criticism of Mr Moon and trade threats are playing into the hands of Mr Kim, whose strategy is to break the US-South Korean alliance, Mr Christopher Hill, a former US ambassador to South Korea and negotiator on North Korea's nuclear programme, said on Twitter on Sunday.

In June, Mr Hill wrote that such a split would "would enable the reunification of the Korean Peninsula on Kim's terms".

One of Mr Moon's main goals is for South Korea's military to complete the transfer of wartime operational control from the US, a legacy of the Korean War in the 1950s. The transition, first agreed in 2007, has been repeatedly delayed by budgetary constraints and regional tensions.

On a visit to the White House in June, Mr Moon and Mr Trump agreed to "expeditiously enable the conditions-based transfer" of control.

South Korea's opposition wants Mr Moon to go even further. Mr Hong Joon Pyo, chief of the main opposition Liberty Korea Party, has pushed for the deployment of US tactical nuclear weapons to test its commitment to defend South Korea. His comments reflect concern that the US could renege on the alliance as North Korea develops the capability to target American cities with nuclear weapons.

While Mr Moon's administration confirmed on Tuesday it still seeks the denuclearisation of the Korean peninsula, Mr Trump's comments have increased speculation that he would be willing to tolerate collateral damage in Seoul to protect the US from a nuclear attack.

Senator Lindsey Graham of South Carolina, a Republican, told NBC News in August that Mr Trump told him that "if thousands die, they're going to die over there".

Said South Korean lawmaker Ha Tae Keung with the conservative-leaning Bareun Party, which controls less than 10 per cent of seats in parliament: "Moon's at least doing better than Trump, who obviously has no idea of what an alliance is. It's hard to predict what Kim will do, but one thing that's clear is that he already saw too much of a gap in the US-South Korea alliance at a time when coordination should have been water tight."

Mr Ha said that while Mr Moon was initially "too naïve," he is now "starting to have some sense of reality".

Mr Trump's rhetoric and threats on trade should boost public support for Mr Moon, according to political science professor Lee Ho Chul at South Korea's Incheon National University.

"While Moon is not having his way with North Korea, that won't immediately bring his approval rate down," Prof Lee said. "Trump's brand of toughness isn't being embraced warmly in South Korea either."

Mr Moon's approval rating is above 70 per cent. That is partly because he played a strong national security hand, even while pleasing left-leaning votes with his push for talks with Pyongyang, according to Mr Christopher Green, senior adviser on the Korean peninsula at the International Crisis Group.

Mr Moon, the son of North Korean refugees, has continued to seek dialogue even as he bolsters South Korea's defences. On Tuesday, he told Russia's Tass news agency he wanted to lay the foundation for peace, though the situation now is "not very good".

"He gets very high approval partly because he is able to bring the conservatives on side with his agenda," Mr Green said. "Moon's strategy cannot possibly survive these kind of circumstances at this time, so what does he do? He beefs up the military side of things."