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Barack Obama arrives in Seoul amid North Korea nuclear fears

PHOTO: US President Barack Obama (C) arrives to lay a wreath at the National War Memorial in Seoul on April 25, 2014.(AFP: Jim Watson)

The US President Barack Obama has arrived in South Korea amid growing signs that North Korea is preparing a nuclear test.

South Korean officials have gathered intelligence that shows all the technical preparations have been completed for Pyongyang to conduct its fourth nuclear test.

A North American think tank says new satellite photos also show additional activity at the Punggye-ri test site that is "probably related to preparations for a detonation".

The latest images suggested increased movement of vehicles and materials near what are believed to be the entrances to two completed test tunnels, the US-Korea Institute at Johns Hopkins University said.

Also visible were probable command and control vehicles intended to provide secure communications between the test site and other facilities.

The report echoed recent warnings from South Korea that the North might be planning a test to coincide with Mr Obama's two-day visit.

In an interview ahead of his arrival in Seoul, Mr Obama warned North Korea could expect a "firm response" if it made "the mistake" of conducting another nuclear test.

Pyongyang is expected to be a major topic on the agenda for talks between Mr Obama and South Korean President Park Geun-hye which is set for Friday afternoon.

Last month North Korea threatened to conduct a new type of nuclear test, referring to an explosion with enriched uranium.

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

Visit overshadowed by ferry tragedy

While in South Korea, Mr Obama will express his condolences over the April 16 ferry accident which has left 300 people - mostly high school students - dead or missing.

The president's trip comes at a sensitive point in the ferry recovery mission, as officials weigh when to bring in cranes and begin cutting up and raising the submerged vessel.

"I know my visit now comes as South Koreans are in mourning and my visit will be an opportunity to express the sympathy of the American people," he said.

"As parents we cannot begin to imagine what all those grieving parents are going through having lost their sons and daughters."

Mr Obama will attend a summit and formal dinner with Ms Park Geun-hye on Friday.

He will also visit some of the 28,500 US troops stationed in South Korea on Saturday, before heading on to Malaysia and the Philippines.