Any launch would send the stuttering talks on denuclearisation into disarray, after a high-stakes second summit between Donald Trump and Kim Jong Un collapsed last month without a deal.

But some analysts suggest the North might be stage-managing activity at certain key sites, to stoke concern and secure "better terms" when the two sides next meet.

Washington wants what administration officials have called a "big deal", with the complete elimination of weapons of mass destruction in return for the dropping of sanctions that have strangled the isolated North's economy.

North Korea favours a more step-by-approach, with Kim proposing dismantling its Yongbyon nuclear complex in exchange for lifting the main sanctions -- a notion Trump refused in Hanoi despite the vaunted "chemistry" between the pair.

"The North could be trying to show the US it can always turn back to aggressive posture by rebuilding missile sites in order to gain leverage in future talks, but without actually firing a missile or rocket," said Cheong Seong-Chang, a senior researcher at the private Sejong Institute.

"It is hard to rule out the possibility of a rocket launch at the Sohae station at this point as North Korea has proven time and time again it can do unexpected things."

- 'All possible scenarios' -

Satellite analysis now indicates increased activity at two key sites -- the Samundong missile research facility and the Sohae rocket launch centre.

Located on the outskirts of Pyongyang, Samundong was built in 2012 to support development of long-range missiles and space-launch vehicles.

As well as developing the Hawsong-15 ICBM, which analysts agree is capable of reaching the whole US mainland, Samundong constructed the long-range rockets that were then transported and successfully launched from the Sohae satellite launch station in 2012 and 2016.

Images of Samundong taken on February 22 showed cars and trucks at the site, as well as rail cars and cranes at a yard, US news outlet NPR has reported.

South Korea is "closely tracking and looking into all activity for possible scenarios including a missile launch" across the border, said Kim Joon-rak, spokesman of the South Korean Joint Chiefs of Staff.

Experts are divided over Pyongyang's plans, but whatever its intentions, a launch would shatter the fragile US-North Korea relationship and revive the angry language that had stoked fears of a military conflict at the start of the Trump presidency.

"This is North Korea's classic brinkmanship on display again," said Lim Eul-chul, professor of North Korean studies at Kyungnam University.

"The North's intention is clear. It wants early resumption of talks with the US, but on better terms."

Veteran North Korea watchers said that any activity is likely to be finely calibrated and could also be intended to send a clear message within the isolated regime.

"Kim could use a launch to demonstrate at the 14th Supreme People's Assembly, probably in mid-April, that he has not been cowed by sanctions," said former US negotiator Joel Wit, now the director of the respected 38 North website.

- 'Pretty disappointed' -

North Korean leader Kim Jong Un agreed last year to shut the Sohae site at a summit with the South's President Moon Jae-in in Pyongyang. Satellite pictures in August suggested workers were dismantling an engine test stand there.

But the Washington-based Center for Strategic and International Studies suggested last week that rebuilding was progressing quickly at the facility.

A moving structure that had been used to carry vehicles to a launch pad on rails has been restored, said 38 North, adding that the work had started before last month's failed meeting in Hanoi.

Chun In-bum, a North Korea expert and a retired three-star general, said it was "too early" to conclude the North was preparing for another rocket launch at Sohae.

However, he said that if the North went ahead, the US would be "forced to react" whether it turned out to be a missile test or a satellite launch.

North Korea has been banned by the UN Security Council from carrying out space launches, as some of its technology was similar to that used for intercontinental ballistic missiles, or ICBMs.

John Bolton, President Donald Trump's hawkish national security adviser, said Sunday the US sees "exactly what they are doing" in regard to possible launch moves by the nuclear-armed state.

"We see it unblinkingly, and we don't have any illusions about what those are," he warned, adding his boss would be "pretty disappointed" if a nuclear-armed state conducted a new missile test.