North Korea walked away from long-running aid for disarmament talks in April in protest at a UN rebuke after it test-fired a long-range rocket.
Weeks later it triggered further international condemnation with its second test of a nuclear weapon.
Speaking to reporters in the South Korean capital, Campbell said the US was willing to resume talks with North Korea, but only "under the right circumstances".
He also said that there should be consequences for North Korea's weapons tests, but said the US would be prepared to offer a "comprehensive package that would be attractive'' to the North if it took "serious and irreversible steps" to disarm.
On Monday, as the talks in Seoul got underway, South Korean coast guard officials said they were preparing new guidelines for boarding and inspecting North Korean ships suspected of carrying banned items.
North Korea has previously warned it would consider such inspections as a declaration of war.
A coast guard official told the Associated Press that the guidelines would call for inspecting North Korean ships travelling in South Korean waters if there is strong evidence that they carry banned items.
Earlier this month, a North Korea ship suspected of carrying weapons including missile parts bound for Myanmar turned back after it had been tracked for several days by a US warship.
|Police in Italy are reported to have seized two yacts ordered by Kim Jong-il [AFP]
The latest UN resolution has toughened sanctions called for in a previous resolution adopted in 2006 after the North's first nuclear test, and also bans countries from exporting luxury goods to North Korea - a clause aimed at the country's ruling elite.
As part of those measures, police in Italy have reportedly seized two luxury yachts that Kim Jong-il, the North Korean leader, had ordered from an Italian shipbuilder.
According to the Libero newspaper, police in the province of Lucca confiscated two yachts waiting to be delivered to Kim in a shipyard in the city of Viareggio.
The paper also said that European financial authorities had confiscated millions of dollars in deposits for the yachts in early April.