WASHINGTON (AFP) - With global tensions spiralling over the North Korean nuclear crisis, US President Donald Trump wants to pile extra pressure on Pyongyang by urging regional allies to dramatically boost spending on high-tech US military gear.

Though existing regulations preclude any sudden changes, Mr Trump's remarks this week open the door to what could amount to a regional military buildup - a move sure to anger China.

"I am allowing Japan & South Korea to buy a substantially increased amount of highly sophisticated military equipment from the United States," Mr Trump said on Tuesday on Twitter.

A day earlier, he told his South Korean counterpart Moon Jae In that Washington is willing to approve arms sales worth "many billions of dollars" to Seoul following Pyongyang's most powerful nuclear test to date.

Tensions are soaring after Pyongyang's test on Sunday (Sept 3) of what it said was a hydrogen bomb designed for a long-range missile.

Declaring that "enough is enough", America's ambassador to the United Nations Nikki Haley said existing measures had not worked and accused North Korean leader Kim Jong Un of "begging for war".

And South Korea's Defence Minister has said it is time to reconsider whether the US should redeploy "tactical" nuclear weapons to South Korea - the reduced-yield warheads were withdrawn in the 1990s.

Experts worry such a move would heighten the risk of a mishap or goad North Korea into conducting a first strike.

Meanwhile, the US is lifting restrictions on South Korean missile payload capabilities, previously restricted to a maximum warhead weight of 500kg. Mr Bruce Klingner, a senior research fellow at the Heritage Foundation, said the limit had been raised to 1,000kg.

Both South Korea and Japan already spend billions of dollars annually on US military technology. Owing to how the US approves arms sales, it is impossible to increase spending overnight.

The State Department's Foreign Military Sales programme is a lengthy process in which bureaucrats vet a request before passing a prospective deal to Congress for approval.

Mr Jim Schoff, a senior fellow at the Carnegie Endowment for International Peace, said Mr Trump cannot circumvent these rules.

"It's not his call to do that," Mr Schoff told AFP. "It's Congress's call in many cases to waive the restrictions and limitations that they themselves have put on the sale of US technology."

Even after Congress approves a sale, it can take the Pentagon months or even years to flow the equipment to the purchasing country.

While Mr Trump's words will have little immediate effect on military sales, his messages may help pressure China into taking a more assertive role in addressing North Korea's weapons programme.

Beijing does not want sophisticated US radars and weapons systems on its doorstep and reacted furiously when the US deployed the Terminal High-Altitude Area Defence (Thaad) system to South Korea.

The US sold arms worth nearly US$5 billion to South Korea between 2010 and 2016, according to an analysis by the Stockholm International Peace Research Institute.

South Korea was the fourth biggest buyer of US arms in that period, behind Saudi Arabia, Australia, and the United Arab Emirates.

And Japan spends roughly US$3 billion (S$4 billion) per year on US defence equipment and weapons systems, Mr Schoff said. Much of that is for high-end US weapons systems such as the F-35 stealth fighter and Global Hawk drones.

Japan could significantly boost its spending on additional F-35s or Aegis Ashore, a land-based adaptation of the maritime Aegis missile-defence system.

Japan already has Patriot batteries, which can stop lower altitude missiles, and SM-3 missiles that can take out short- to intermediate-range ballistic missiles flying at higher altitudes.

Some experts are calling for the US and its allies to shoot down future North Korean missiles.

The technology is not perfect but the Pentagon has demonstrated it can hit ICBM and intermediate-range missile targets.

An attempted shoot-down would "put the burden of risk on North Korea in challenging the United States and its allies and of escalating any potential crisis", wrote Mr Evans Revere and Mr Jonathan Pollack of the Brookings Institution.

Mr Schoff said any attempt to redeploy tactical nuclear weapons to South Korea was pointless, as it would amount to exposing bombs that are currently well hidden in submarines or elsewhere.

"Why would you want to take the nukes off those hard-to-find platforms, put them in a bunker with a big 'hit me here' sign on it?" he said.

Mr Klingner, of the Heritage Foundation, said Mr Trump's messages may have been more aimed at reassuring South Korea rather than any meaningful shift in how weapons are sold.

"In a lot cases with (South) Korea, it's: 'We want you to do something to reassure us,' and sometimes the specifics don't matter," he told AFP.