South Korea's military is carefully monitoring North Korea's nuclear and missile facilities after it said new activity has been detected at the intercontinental ballistic missiles plant.

The US and South Korean militaries are maintaining close intelligence coordination over developments at the North’s missile research centre in Pyongyang and a separate long-range rocket site, Seoul’s Defence Ministry spokeswoman Choi Hyun-soo said on Thursday.

She did not elaborate, but South Korean media reported that movement of cargo vehicles were spotted recently around the facility at Sanumdong in Pyongyang, which produced North Korea’s first intercontinental ballistic missiles (ICBMs) capable of reaching the US.

Spy chief Suh Hoon told politicians briefed by the South’s National Intelligence Service he viewed the activity as missile-related, the JoongAng Ilbo newspaper said.

It also quoted Mr Suh as saying the North continued to operate its uranium enrichment facility at the main Yongbyon nuclear complex after the first summit between Donald Trump and North Korean leader Kim Jong-un in Singapore last June.

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It comes after a second summit between Mr Trump and Mr Kim broke down last week in the Vietnamese capital of Hanoi over disagreements about limiting the North's nuclear programme in return for US sanctions relief.

On Tuesday, two US think-tanks and the South Korean spy agency reported work was underway to restore part of the North’s Sohae Satellite Launching Station, even as the Hanoi meeting took place.

North Korea began dismantling a missile test stand at Sohae last year, in line with a pledge Mr Kim made at his first summit with Mr Trump.

“I would be very disappointed if that were happening,” Mr Trump told reporters in the Oval Office, when asked if North Korea was breaking a promise.

“It’s too early to see ... It’s a very early report. We’re the ones that put it out. But I would be very, very disappointed in Chairman Kim, and I don’t think I will be, but we’ll see what happens. We’ll take a look. It’ll ultimately get solved.”

North Korea's foreign minister disputes President Trump's explanation for summit collapse

Earlier this week, satellite imagery from Planet Labs Inc analysed by the Center for Nonproliferation Studies at the Middlebury Institute of International Studies in California showed activity at Sohae from 23 February up until Wednesday.

The Washington-based 38 North said its latest photos from Wednesday indicated the rail-mounted transfer building used to move rockets at the site was now complete, and said cranes had been removed from the launch pad and the transfer building had been moved to the end of the pad.

On Tuesday, Mr Trump’s national security adviser, John Bolton, warned new sanctions could be introduced if North Korea did not scrap its weapons programme.