North Korea must return to a "diplomatic path", the top US envoy for the country has said, following Pyongyang's blitz of missile launches and growing signs it is resuming nuclear testing.

North Korea has carried out more than a dozen weapons tests so far this year, the latest one over the weekend - a short-range test that state media claimed would enhance the "efficiency in the operation of tactical nukes".

US Special Representative for North Korea Sung Kim arrived in Seoul for a five-day visit and met his South Korean counterpart Noh Kyu-duk.

After talks, the two envoys jointly condemned "recent escalatory actions" by Pyongyang, including what they said were "at least three" launches of intercontinental ballistic missiles (ICBMs).

"We agreed on the need for a strong response to the destabilising behaviour we have been from the DPRK," Mr Kim told reporters, using the acronym for North Korea's official name.

"We also discussed how to respond to future DPRK actions, including a possible nuclear test," he added.

Mr Kim reiterated that the United States has offered to meet North Korea "anywhere without any preconditions", saying that Washington has not "closed the door on diplomacy".

"I once again call on Pyongyang to pursue a diplomatic path with us," he said. "I want to make clear that we have no hostile intent towards the DPRK."

North Korea has so far rebuffed offers of talks, blaming what it said were the United State's hostile policies.

Mr Kim's visit comes as South Korea and the US kicked off a nine-day annual joint military drill. Such exercises have always infuriated North Korea, which calls them a rehearsal for war.

The allies regularly stage military exercises, but they have been scaled back in recent years as outgoing President Moon Jae-in made efforts to facilitate nuclear talks with North Korea.

"This training is a defensive command post training using computer simulation, and there is no real military manoeuvre training," Seoul's Joint Chiefs of Staff said yesterday.

North Korea had paused long-range and nuclear tests while leader Kim Jong Un met then-US president Donald Trump for a bout of doomed diplomacy, which collapsed in 2019. Talks have since stalled.

Last month North Korea test-fired an ICBM at full range for the first time since 2017.

Recent signs of new activity at North Korea's key nuclear testing site have also raised concerns over the possible resumption of nuclear testing.

South Korean officials have said North Korea could stage a military parade or carry out a weapons test on or around 25 April, the anniversary of the founding of the Korean People's Army.