North Korean leader Kim Jong Il made his first known public appearance since his nation's nuclear test, official media reported Wednesday, amid concerns that the regime was readying a second detonation.

Kim, accompanied by top Communist Party officials and military officers, attended a performance of songs praising him, the official Korean Central News Agency reported.

The exact date of the performance was not specified, but Kim most likely attended the event Tuesday evening on the 80th anniversary of the "Down-with-Imperialism Union" — a political platform on which the ruling party was built.

A second nuclear test was not ruled out by Ri Gun, director-general of the North Korean Foreign Ministry's American affairs bureau, in an interview with ABC News in Pyongyang that was broadcast Wednesday.

Asked if there would be a second test, he said: "Even if there's a nuclear test, that is natural, so don't have to care much about this issue. The test itself ... will be natural."

The North Korean performers "sang high praises of the immortal exploits" of Kim and his late father, Kim Il Sung, KCNA reported. The North Korean leader "waved back to the enthusiastically cheering artists and audience and congratulated them on their successful performance."

There was no mention of a nuclear test in the report. On Tuesday, North Korea said U.N. sanctions imposed over its nuclear test amounted to a declaration of war and threatened to "deal merciless blows" against anyone who violated its sovereignty. China and South Korea both warned the North not to aggravate tensions.

Concern over a second test stems partly from new satellite imagery showing increased activity around at least two other North Korean sites, said a senior U.S. Defense official.

The activity, started a number of days ago, included ground preparation at one site and construction of some buildings and other structures, said the official, who spoke on condition of anonymity because it involved intelligence gathering. He said that although the purpose of the structures is unclear, officials are concerned because North Korea has left open the possibility of another test.