The United States and South Korea agreed Thursday to delay their joint military exercises until after the Winter Olympics next month, in an apparent move to de-escalate tensions with Pyongyang.

The announcement came just hours after US President Donald Trump said high-level talks set for next week between North and South Korea were "a good thing".

Tensions have spiralled in recent months after North Korea held multiple missile launches and its sixth and most powerful nuclear test -- purportedly of a hydrogen bomb.

Trump has also traded personal insults with his North Korean counterpart Kim Jong-Un, rattling regional allies.

But the last few days have witnessed a rare softening of tone on both sides of the demilitarised zone after Kim offered an olive branch to Seoul during a New Year's speech, saying he was willing to send a team to next month's Winter Olympics in the South.

The tentative rapprochement took a further step Thursday after South Korean president Moon Jae-In spoke to Trump by telephone with both agreeing to suspend joint military drills, a regular source of Pyongyang's ire.

"The two leaders agreed to de-conflict the Olympics and our military exercises so that United States and Republic of Korea forces can focus on ensuring the security of the Games," the White House said in a statement.

Moon's office said the South Korean president told Trump that delaying the exercises would help ensure the success of the Winter Olympics -- being hosted by the South next month in Pyeongchang -- "in case the North does not make any more provocations".

US Defence Secretary Jim Mattis later said the delay was for practical, rather than political, reasons.

"We have at times changed the timelines on these (drills) for any number of reasons, so for us, this is the normal give and take that we have," he said, noting that the Olympics are South Korea's biggest event in terms of international tourism.

He added that the drills, known as Foal Eagle, would be conducted sometime after the Paralympics, which end on March 18.

- New Year rapprochement -

After a year that saw tensions on the Korean peninsula spike to their worst levels in years, 2018 has begun on a tentatively warmer note with Seoul responding positively to Kim's New Year speech.

On Wednesday the two Koreas restored a cross-border hotline that had been shut down since 2016.

They also agreed to hold high-level talks next week -- the first since 2015 -- which will focus on "matters of mutual interest", including the North's participation in the Winter Olympics.

Mattis said the talks were the result of international pressure, pointing to successive United Nations Security Council votes against the North.

But North Korea's young leader has shrugged off a raft of new sanctions and heightened rhetoric from Washington as his regime drives forward with its weapons programmes, which it says are meant to defend against US aggression.

While Trump called the talks a "good thing" in a tweet on Thursday, his ambassador to the United Nations, Nikki Haley, struck a much more cautious tone earlier in the week.

"We won't take any of the talks seriously if they don't do something to ban all nuclear weapons in North Korea," she said Tuesday.

A State Department spokeswoman also warned that Pyongyang's olive branch may be an attempt to "drive a wedge of some sort" between Washington and Seoul.

The White House statement announcing the suspension of drills said both Trump and Moon "agreed to continue the campaign of maximum pressure against North Korea and to not repeat mistakes of the past".

Pyongyang's missile and nuclear tests has seen the isolated state slapped with painful new sanctions that even its key ally China have backed.

But South Korea and Washington's regular joint military drills have also been criticised by some as adding to regional tensions, particularly by Beijing and Moscow who have both called for them to be suspended.

Russia's deputy foreign minister Sergei Ryabkov on Thursday said he "welcomed" the halting of drills during the Olympics.

News agency RIA Novosti quoted him as saying that Moscow "observes with satisfaction" that their calls to halt the manoeuvres have been "taken into account".

Kim's New Year address also included a warning to the US that he has a "nuclear button" on his table, prompting a furious response from Trump via Twitter that Washington's nuclear button was "much bigger and more powerful".

The tweet generated responses both on Twitter and from analysts largely of scorn and alarm. Mattis declined to address the president's tweet.

"My job as the secretary of defence is to make certain that we have forces ready to defend this country," Mattis said.