US President Joe Biden and his South Korean counterpart Yoon Sok Yul this week issued a warning to North Korea that any nuclear attack on its part would lead to the "end" of the Pyongyang regime.

This threat comes as Sok Yul is on a six-day state visit to the United States, where he discussed with Biden strengthening the US security umbrella for South Korea in the face of North Korea's increased ballistic missile tests.

But what is the significance of this stark statement? Here's a look at what we know:

Washington's declaration strengthens the US security umbrella for South Korea.

These security measures include sending an American nuclear submarine to South Korean waters, which has not happened since the 1980s, and other measures, including sharing more information in the event of a North Korean attack.

But there are no plans to station US nuclear weapons in South Korea, and some analysts question the practical value of the announcement.

"That North Korea fears a strategic nuclear submarine equipped with ballistic missiles with a range of 7,400 km is questionable," Cheong Seong-chang, of the Center for North Korea Studies at the Sejong Institute, told AFP.

He explained that the "very long" range of the submarine missiles meant that they might not be able to hit North Korea if it was in South Korean waters.

For his part, Leif-Eric Easley, a professor at the University of Iowa in Seoul, told AFP that there is no doubt that this visit "represents a very big indication of the level of US-South Korean relations with the expansion of security, economic and cultural cooperation."

US officials described the new arrangement as similar to moves last seen when Washington oversaw Europe's defense against the Soviet Union.

Yoon is trying to reassure an anxious South Korean public about the US's commitment to what it calls "extended deterrence," where US assets, including nuclear weapons, are used to prevent attacks on allies.

Opinion polls showed that a majority of South Koreans believe the country should develop its own nuclear weapons. Yoon previously hinted that Seoul might resort to this option.

Of course not, and this may cause problems, according to experts.

"One thing is clear: There is a tacit agreement that Seoul will not go nuclear," said Sue Kim, director of policy at LMI Consulting Group and a former CIA analyst.

"Seoul's nuclear ambitions have been put to rest," he added.

For his part, Ji-wook Shin, a Korean expert and professor of sociology at Stanford University, told AFP that the announcement is "a step forward."

"I don't think this will be enough to satisfy the South Korean people, who are increasingly demanding that Seoul develop its own nuclear weapons," he added.

Experts say the close cooperation between its two archenemies, Washington and Seoul, is a source of concern for the Kim Jong-un regime and there may be more missile launches to show that.

The retired South Korean army general, Chun In-bum, told AFP that in public, "North Korea will underestimate the importance of the message of reassurance sent by the United States regarding its nuclear deterrence."

But behind closed doors, "they will get the message: if they use nuclear weapons, it will be the end of the regime."

Having spent decades and using a large portion of the impoverished nation's gross domestic product developing banned nuclear weapons programmes, Kim is not going to change course, experts say.

"It is unlikely that North Korea will give up its nuclear weapons by giving in to these threats," Yang Moo-jin, president of the University of North Korean Studies in Seoul, told AFP.

Carl Friedhoff of the Chicago Council on Global Affairs (Chicago Council on Global Affairs) told AFP that the main problem with Washington's declaration is not the agreement itself but rather the political imbalance of the United States, which means it may be worthless after the upcoming US presidential elections.

He explained that the possible return of former US President Donald Trump to power is likely to lead to "very serious discussions" in Seoul.

For the US-South Korean relationship, he added, "the biggest challenge is something the alliance has no control over: the domestic politics of the United States."

"There is real concern in Seoul about the return of the Republican Party, especially Trump, to the White House. If he wins the 2024 elections, this could lead to a shift in the relationship" between the two countries, he added.