The United States and South Korea are discussing how to implement last week's UN Security Council resolution imposing sanctions on North Korea.
The two countries' top diplomats warn of grave consequences, if North Korea does not halt its nuclear testing.
U.S. Secretary of State Condoleeza Rice and South Korean Foreign Minister Ban Ki-moon said Thursday they wanted to send a "clear message" to North Korea that it must give up its nuclear weapons.
Speaking together to reporters here in Seoul, they expressed a shared belief that Pyongyang must return unconditionally to multinational talks aimed at ending its nuclear programs.
Rice and Ban made it clear, however, that they were still searching for ways to implement the sanctions that the UN imposed after last week's North Korean first-ever nuclear test.
The resolution, among other things, calls on members to prevent North Korea from proliferating nuclear weapons and technology. That could include searching North Korean cargo suspected of containing nuclear materials.
But Rice emphasized, as she did Wednesday during a visit to Tokyo, that the resolution did not call for a blockade of North Korea. She said the United States would not try to exacerbate an already tense situation.
"The idea that somehow we would want Resolution 1718 to be implemented in a way that escalates tensions on the Korean Peninsula, or on the high seas for that matter, simply could not be more wrong. What we want is effective implementation."
She says scrutinizing North Korean cargo could be accomplished mainly in ports, with the help of intelligence and through technologies, such as radiation detectors, rather than by interdictions on the high seas.
North Korea's neighbors have expressed concern that such interdictions could lead to clashes.
South Korea's Ban, who will soon take over as secretary-general of the United Nations, warned North Korea not to conduct further nuclear tests.
He says, if the North does more testing, there will be more serious consequences.
Ban did say South Korea will re-evaluate two projects it has funded in the North, a tourist resort and a special economic zone, to see if their continued operation complies with the U.N. resolution.
The two diplomats, however, generally avoided providing specifics about what acts their two countries might take, individually or collectively.
Rice said the resolution had been passed relatively quickly following the North Korea test of October 9th, and nations were still discussing how best to implement the resolution's provisions.
Rice is scheduled to travel to Beijing on Friday, and then to Russia.