The United States is closely watching North Korea for signs of a possible missile launch or nuclear test in the coming days that officials are referring to as a "Christmas surprise". Any such action would likely stifle progress in a major US foreign policy initiative to renegotiate a denuclearisation deal with leader Kim Jong Un. Efforts are being made by the Trump administration to draw North Korea back to the table, though the US has rejected an end-of-year deadline set by Pyongyang to make concessions in the negotiations. North Korea has described recent tests as "crucial" - the US concerned that a supposed engine trial earlier this month could be part of plans to develop a space launch vehicle or a long-range missile.

Amid fears of Kim Jong Un acquiring an intercontinental ballistic missile (ICBM) capable of reaching the US, experts are on high-alert. Anthony Wier, a former State Department official who tracks nuclear disarmament for the Friends Committee on National Legislation, said: "North Korea has been advancing. It has been building new capabilities. "As long as that continues, they gain new capabilities to try new missiles to threaten us and our allies in new ways."

North Korea launched a 'Super Large Multiple Rocket' system in November (Picture: KCNA).

Senior officials in North Korea warned of a possible "Christmas gift" in early December, warning that the US was running out of time to salvage nuclear negotiations. It was explained that US action, or lack of action, would dictate which "gift" it gets from the North. Victor Cha, a Korea expert at the Centre for Strategic and International Studies, said a review of the possible launch sites in North Korea shows that they are a "basically ready to go". He said the expected launch could be a test of a sea-based ballistic missile, difficult to locate, or a solid-fuel rocket - providing a quick-fuelling capability that would give others less time to react.

Meanwhile, US Defence Secretary Mark Esper said he is familiar with the "bluster" from the Korean Peninsula. He said: "We need to get serious and sit down and have discussions about a political agreement that denuclearises the peninsula. "That is the best way forward and arguably the only way forward if we're going to do something constructive." Speaking earlier this week in South Korea, US Special Representative for North Korea Stephen Biegun said the United States is "fully aware of the strong potential for North Korea to conduct a major provocation in the days ahead".