A senior US official said on Thursday that China had sent a special envoy to China. "I'm pretty convinced that the Chinese will have a very strong message about future tests," he said.
China's foreign ministry declined to confirm that Chinese state councillor Tang Jiaxuan was currently visiting Pyongyang.
The US official, who asked not to be identified, was travelling with Condoleezza Rice, the US secretary of state, as she arrived in Seoul from Tokyo seeking South Korean help in pressuring its northern neighbours into ending its nuclear weapons programme.
However as Rice arrived in Seoul, one of the North's top diplomats said that it would be "natural" to follow up his country’s underground explosion on October 9 with a second nuclear test.
"We don't have to care much about this issue," Li Gun, the deputy head of North Korea's foreign ministry, told the US television network ABC.
Asked if US officials should not be surprised by another test, Li said: "That's right, yes."
"It is certain that the North will conduct three or four additional nuclear tests in the future"
Chung Hyung-keun, South Korean MP
A South Korean MP and parliamentary intelligence committee member, Chung Hyung-keun, said the North could be preparing three or four more nuclear tests.
"Checking indications coming from intelligence agencies of different countries, it is certain that the North will conduct three or four additional nuclear tests in the future," he told SBS radio.
In Tokyo Rice had assured Asian allies of the US’ commitment to defend them.
In Seoul Rice will meet the South Korean foreign minister Ban Ki-moon, who has been named the next secretary-general of the UN and they will later be joined by their Japanese counterpart, Taro Aso.
South Korea is to tighten inspection of all cargo heading to the North and plans to cut funding to a mountain resort in the North which is open to South Koreans to visit and which the US says is a cash cow for leaders in Pyongyang.
Japan has been a traditional target of Pyongyang's animosity and debate has increased in the country over whether to acquire nuclear arms.
Aso:Japan has no intention of
acquiring nuclear arms
But Aso reiterated that Tokyo had "absolutely no intentions now of preparing to possess nuclear weapons".
Rice travels to Beijing on Friday to push for strong enforcement of the UN sanctions from North Korea's biggest trading partner, particularly the inspection of North Korean cargo to intercept weapons and weapons parts.
China is the closest North Korea has to an ally but backed the UN resolution and has said it will carry out cargo inspections but not conduct searches at sea.
A Reuters correspondent in Dandong, on China's border with North Korea, said Chinese customs agents were checking papers and vehicles but otherwise there was no increased scrutiny.