The United States is ready to hold talks with North Korea if the conditions are right but will also press U.N. sanctions to punish Pyongyang for its nuclear and missile tests, a senior envoy for Asia said on Saturday.

North Korea, which tested a nuclear device in May, has said it was boycotting often stalled six-country talks on ending its nuclear arms program and will expand its nuclear arsenal in order to defend itself against a hostile United States.

We have to be clear that under the right circumstances, we should be prepared to sit down with North Korea if they would abandon their nuclear ambitions, U.S. Assistant Secretary of State Kurt Campbell said upon arriving in Seoul ahead of talks with South Korean officials.

There have to be consequences for these provocative actions so the United States is working in the international community, with the United Nations, to put forward a robust set of sanctions and unilateral actions that are designed to send a clear message to Pyongyang, he said.

The last round of the disarmament-for-aid talks among the two Koreas, China, Japan, Russia and the United States was held in late 2008 as the term of former U.S. President George W. Bush was nearing its end.

The U.N. Security Council on Thursday expanded the list of North Korean bodies and individuals under sanctions for nuclear and missile activities, adding its atomic energy agency and two of its top officials.

The announcement of the list followed a month of committee haggling after the Security Council expanded U.N. sanctions against North Korea in a June 12 resolution that responded to a nuclear test Pyongyang carried out on May 25.

The sanctions are aimed at cutting off North's arms trade, a vital source of hard currency for the cash-short state.

North Korea has rattled the security in the region with the nuclear test, threats to attack the South and the test-launch of seven ballistic missiles earlier this month in defiance of a U.N. resolution barring the reclusive state from firing ballistic missiles.

South Korean government officials said the military moves were aimed at building internal support for leader Kim Jong-il, 67, who is reportedly in poor health and wants to prepare for his youngest son to succeed him in Asia's only communist dynasty.