A top envoy said the U.S. remains ready for talks with North Korea, while Washington warned of aggressive sanctions against the North unless it returns to stalled multinational talks on ending its nuclear programs.

"Under the right circumstances, we'd be prepared to sit down with North Korea if they would abandon their nuclear ambitions," Kurt Campbell, the assistant secretary of state for East Asia and Pacific affairs, told reporters after arriving in South Korea on Saturday.

It wasn't immediately clear whether Campbell was referring to direct bilateral meetings, which North Korea has sought, or talks within a six-nation framework that had been ongoing before North Korea abandoned the process in April.

Campbell also said there have to be "consequences" for North Korea's recent provocative actions.

Before flying to Seoul, he met with Japanese officials in Tokyo and agreed to increase efforts to encourage North Korea to return to the disarmament talks.

Campbell was to meet his South Korean counterpart later Saturday and Seoul's foreign minister and its top nuclear envoy on Monday.

The trip to the Asian allies comes as the U.S. is moving to enforce U.N. as well as its own sanctions against North Korea for its May 25 nuclear test.

In Washington, State Department spokesman Philip Crowley urged North Korea on Friday to return to the nuclear talks and begin taking irreversible steps toward denuclearization.

He also warned that the U.S. will continue pressuring North Korea into giving up its nuclear programs, saying Washington is "aggressively implementing" the U.N. sanctions.

The six-nation talks came to a halt in April when North Korea withdrew to protest a U.N. statement condemning what North Korea said was a satellite launch. The U.S., and its allies said the launch was actually a long-range missile test.

The talks -- which involve the two Koreas, the U.S., Japan, China and Russia -- were last held in Beijing in December.

Kim Yong Nam, North Korea's No. 2 official, said this week that his country is not ready to resume nuclear disarmament talks because the U.S. and its allies do not respect its sovereignty.

North Korea's "nuclear weapon is not for invading or threatening others but is war deterrence for defending the peace and security on the Korean peninsula," he said in a speech Wednesday at the Nonaligned Summit in Egypt, the North's official Korean Central News Agency reported Saturday.

He also said his country "is opposed to a nuclear war, nuclear arms race and nuclear proliferation."

In Tokyo, Campbell and Japanese officials agreed to strengthen the U.S. defense of Japan against any nuclear attack by North Korea -- Japan's top security concern.

The U.S. also is seeking to take steps to ensure that North Korea is less able to spread nuclear technology and weapons elsewhere in the world, Crowley said.