New satellite images show more work under way at North Korea's nuclear test site but give no indication of when a test may occur, a US-based specialist website reported Saturday.

There is widespread speculation that the North may carry out another atomic weapons test -- its third -- following international criticism of a failed rocket launch on April 13.

The 38 North website carried satellite photos of the Punggye-ri site near the east coast, saying they seem to indicate that "the North Koreans are proceeding as if the test go-ahead decision has already been made.

"The imagery does not, however, give any indication of when that detonation may take place," said the website, a project of the US-Korea Institute at Johns Hopkins School of Advanced International Studies.

It said the pictures taken by a private company on April 18 are the latest in a series showing continuing preparations for a test.

Photos taken on March 8 showed soil and other material excavated from the test tunnel. Overall, it said, thousands of tons of rubble have been excavated at the site covering 4,000 square metres (around one acre).

The latest imagery shows a train of mining carts on top of the soil piles and random unidentified structures or objects on or near the piles, 38 North said.

Seoul's deputy defence minister Lim Kwan-Bin was separately quoted as saying that South Korean and US defence officials see a "very high" chance of another test.

"The assessment of South Korea and the US is that chances are very high that North Korea will carry out a nuclear experiment," Yonhap news agency quoted him as saying Friday in Washington after two days of meetings with senior Pentagon officials.

"(We) can't predict the specific timing of it, but it is believed to be possible any time."

Lim said he had no firm evidence but cited Pyongyang's record of conducting nuclear tests following criticism of long-range missile launches in 2006 and 2009.

"There are many opinions that there is a high possibility that it will go ahead with a nuclear test to make up for the failed missile launch," he was quoted as saying.

South Korean and US officials reaffirmed they would stage a tough response to any additional provocations by the North, Lim said. The US bases 28,500 troops in the South.

The North's official news agency said leader Kim Jong-Un has observed a military exercise involving tanks, planes and artillery, amid continuing threats by Pyongyang against Seoul.

Kim, supreme commander of the 1.2 million-strong military, "guided" the drill staged to mark the 80th anniversary of the armed forces, it reported without giving a date for the exercise.

"He called for mercilessly wiping out the enemies with the arms of justice and revenge if they dare provoke," it said.

The North has threatened "sacred war" against the South's conservative government in retaliation for perceived insults during Pyongyang's commemoration this month of the centenary of the birth of founding leader Kim Il-Sung.