North Korea has long protested joint military drills, which it condemns as preparations for invasion, and has set Washington an end-of-year deadline to come up with a new offer in deadlocked negotiations on its weapons programmes.

The US and South Korea last year cancelled several joint drills in the wake of the Singapore summit between President Donald Trump and the North's leader Kim Jong Un, but were due to carry out a combined air exercise later this month.

The joint air drills will now be postponed, Esper said, days after hinting that the option was on the table.

"We have made this decision as an act of goodwill to contribute to an environment conducive to diplomacy and the advancement of peace," he told reporters on the sidelines of a meeting of defence ministers in Thailand's capital.

He urged Pyongyang to return to negotiations and "demonstrate the same goodwill as it considers decisions on conducting training, exercises and testing".

Esper said the decision to delay the drills was not a concession but an effort to create "some more space" for diplomats to strike an agreement.

His South Korean counterpart Jeong Kyeong-doo said there was no timetable for resuming the exercises.

"It will be a part of our ongoing consultation and we will decide through that close coordination between the two sides," Jeong said.

Pyongyang has carried out a series of missile tests in recent months, including one launched at sea which it said was fired from a submarine -- a potential strategic game-changer.

It has repeatedly demanded that the combined exercise is scrapped, and recently said holding the drills would be an "undisguised breach" of the Singapore summit declaration.

Negotiations have stalled since the follow-up Hanoi summit between Kim and Trump broke up in February, with disagreement over sanctions relief and what the North would be willing to give up in return.

Working-level talks restarted in Sweden in October only to break down quickly, with the North blaming the US for not giving up its "old attitude".

Pyongyang welcomed reports that the joint drills would be postponed in a Sunday statement by an unnamed foreign ministry spokesman, published in state media outlet KCNA.

But the spokesman also lashed the US for its "hostile policy" towards North Korea and blamed Washington for a recent UN meeting that criticised human rights violations under the isolated regime.

"The US has no intention to sincerely work with us towards the settlement of issues. Therefore, we have no willingness to meet," the official said, according to the KCNA report.

- 'Work together' -

Esper was in Bangkok after meeting with South Korean officials during a two-day trip to Seoul.

There he urged the South to drop its plan to end a military intelligence sharing agreement with Japan, which Washington considers vital to security co-operation in the face of the North and ultimately China.

In the latest manifestation of the neighbours' dispute over wartime history, Seoul announced in August it would terminate the pact, saying it did not serve national interests. The agreement expires at midnight on November 22.

On Sunday Esper called for both countries to "work together" to overcome their differences.

"The only people that benefit from friction between Seoul and Tokyo are Pyongyang and Beijing," he said.

Seoul and Tokyo are both major US allies, democracies and market economies faced with an overbearing China and the wayward North.

But their relationship continues to be heavily affected by Japan's colonial rule of the Korean peninsula from 1910 to 1945.