North Korea has apparently sealed a tunnel at its nuclear test site, a government official said Thursday ahead of US President Barack Obama's arrival in Seoul for a two-day visit.

South Korea's defence ministry warned earlier this week that the North could be preparing to carry out a fourth nuclear test, citing increased activity at the Punggye-ri site.

"It is believed that North Korea has placed detonator, fissile material and measurement devices in the tunnel and sealed it," the official, who declined to be identified, said Thursday in a briefing for foreign news media.

He said the information was based on unspecified intelligence reports.

Experts are divided over whether a test is imminent and the defence ministry warning on Tuesday came with the caveat that Pyongyang might be pretending to prepare a detonation to raise tensions during Obama's visit.

The government official said he could not say precisely when the tunnel had been sealed.

North Korea has conducted three nuclear tests -- in 2006, 2009 and 2013 -- all at the Punggye-ri site.

Pyongyang has described Obama's Asia trip as a "dangerous" move that would escalate military tension and bring the "dark clouds of a nuclear arms race" over the Korean peninsula.

Speaking in Tokyo on Thursday, Obama said North Korea had been an "irresponsible actor" on the international stage for several decades.

With the help of Japan, South Korea and China, Obama said "more and more pressure" could be brought to bear on Pyongyang to give up its nuclear ambitions.