SEOUL (AFP) - North Korean leader Kim Jong Un has chided his soldiers, telling them to be ready for "impending conflict with the United States," Pyongyang media reported on Saturday as satellites showed a nuclear test could be near.

The report comes as US President Barack Obama finishes up a two-day visit to South Korea, where he warned the North it faced tougher sanctions if the underground detonation went ahead.

It also comes after Pyongyang claimed it had been holding a young American for two weeks. Pyongyang said late on Friday that it had been holding Miller Matthew Todd, 24, since April 10 because of his "rash behaviour" while passing through immigration.

Mr Kim, the supreme commander of the North's 1.2-million-strong armed forces, often visits military units to deliver on-the-spot "guidance" on ways to strengthen preparedness. He usually lavishes them with praise and presents gifts such as rifles or binoculars as symbols of their vigilance.

But after watching a shelling drill by an artillery sub-unit on Friday, he upbraided soldiers for their lax approach, the Korean Central News Agency (KCNA) said.

"Dear Supreme Commander Kim Jong Un said nothing is more important than preparing for combat now, in the face of an impending conflict with the United States", KCNA reported. North Korean state media regularly carries colourfully-phrased warnings that the isolated state is on the verge of war.

Pointing at a map, Mr Kim ordered the unit to establish a firing position and start the shelling exercise, the agency said, without revealing the location. "Watching the drill, he severely criticised the sub-unit for failing to make good combat preparation", citing the time it had taken to deploy, it said.

He blamed a lack of enthusiasm over training among the sub-unit's commander and his superiors. "The minds of the commanding officers of this sub-unit and relevant unit seem to be away from the battlefield", he said.

It is unusual for the agency to carry direct quotes from Mr Kim.

"Of course, they might do sideline jobs for improving service personnel's living conditions and do their bit in building a rich and powerful nation. However, they should always give priority to combat preparations", he said.

Speaking in Seoul on Friday after satellite imagery revealed the North was advancing preparations for a nuclear test, Mr Obama warned it of sanctions with "more bite" unless it fell into line.

North Korea has conducted three nuclear tests, in 2006, 2009 and 2013.