SEOUL - Three US aircraft carriers, accompanied by South Korean warships, launched a joint naval drill Saturday in a fresh show of force aimed at North Korea, whose nuclear ambitions have dominated US President Donald Trump's ongoing tour of Asia.

The four-day exercise in the western Pacific involves three flattops - USS Ronald Reagan, USS Nimitz and USS Theodore Roosevelt - and seven South Korean vessels including three destroyers. It is the first such triple-carrier drill in the region for a decade.

"The exercise is aimed at enhancing deterrence against North Korea's nuclear and missile threats and showing off preparedness to fend off any provocative acts by the North," a South Korean defence ministry spokesman said.

Nuclear-armed North Korea regularly denounces such military drills as rehearsals for invasion and sometimes conducts its own military manoeuvres or missile tests in response.

The US warships will carry out air defence drills, sea surveillance, defensive air combat training and other operations, the US Navy said.

The exercises come on the heels of Trump's visits to Tokyo, Seoul and Beijing this week, which were dominated by the question of how to counter Pyongyang's nuclear weapons threat.

At a summit in Seoul, Trump and South Korean President Moon Jae-In agreed to increase the deployment of US military assets around the Korean peninsula to step up pressure on the North.

And in a speech to the South Korean parliament, Trump warned North Korea not to underestimate the United States, while offering leader Kim Jong-Un a better future if he gives up his nuclear ambitions.

The North, which this year carried out its sixth nuclear test, generating by far the largest yield to date, has fired dozens of missiles in recent months - all in defiance of multiple UN sanctions.

Two have overflown key US ally Japan, and the regime has claimed that it now has the technology to mount a nuclear warhead on a missile that can travel as far as the US mainland.

There has been speculation that Kim would order another missile test to coincide with the US president's marathon visit to the region which ends on Tuesday.

Since becoming president, Trump has engaged in an escalating war of words with Kim, trading personal insults and threats of military strikes and raising concerns about an outbreak of hostilities.

His remarks during his Asian tour - including references to Kim's "twisted fantasies" - have been tempered by calls for Pyongyang to come to the negotiating table and seek a diplomatic route out of the crisis over its nuclear arsenal.

During talks in Beijing on Thursday, Trump urged Chinese leader Xi Jinping to do more to rein in the Pyongyang regime, warning that "time is quickly running out".

The US administration thinks China's economic leverage over North Korea is the key to strong-arming Pyongyang into halting its nuclear weapons and missile programmes.

Xi said the two countries reiterated their "firm commitment" to the denuclearisation of the Korean peninsula and the implementation of UN resolutions.

Though China has backed UN sanctions, US officials want Chinese authorities to clamp down on unauthorised trade along the North Korean border.

But experts doubt China will take the kind of steps that Trump wants, such as halting crude oil exports to the North.

Beijing fears that squeezing Pyongyang too hard could cause the regime to collapse, sending an influx of refugees across its border and placing US forces on its doorstep.