US spy agencies are seeing signs that North Korea is constructing new missiles at a factory that produced the country's first intercontinental ballistic missiles capable of reaching the US, according to officials familiar with the intelligence.

Newly obtained evidence, including satellite photos taken in recent weeks, indicates that work is underway on at least one and possibly two liquid-fueled ICBMs at a large research facility in Sanumdong, on the outskirts of Pyongyang, according to the officials, who spoke on the condition of anonymity.