North Korea has denounced the UN Security Council resolution which imposes weapons and financial sanctions over its claimed nuclear test.
North Korea's UN ambassador said the Council had behaved like "gangsters" and any further US pressure would be seen as "a declaration of war".
The resolution was passed unanimously but China has expressed reservations.
US Secretary of State Condoleezza Rice is set to visit the region next week to encourage implementation of sanctions.
The incoming UN Secretary General, South Korea's Ban Ki-moon, has told the BBC that he is prepared to travel to Pyongyang if necessary to help to defuse the crisis.
'Swift and tough'
The Security Council on Saturday approved unanimously Resolution 1718 which imposes weapons and financial sanctions but is not backed by the threat of military force.
North Korea's UN envoy Pak Gil Yon condemned the move before storming out of the meeting in New York.
China backed the text despite "reservations" on cargo checks
"It is gangster-like for the Security Council to have adopted today a coercive resolution while neglecting the threats and moves for sanctions and pressure of the United States."
The decision to impose sanctions has been broadly welcomed internationally but correspondents say the intense diplomatic bargaining over the scale and nature of the sanctions was clearly evident.
China said it had "reservations" about provisions for cargo checks on North Korean ships and Russia said the sanctions should not be viewed as indefinite.
China and Russia have been worried that the cargo inspections permitted in the resolution could spark naval confrontations with North Korean boats.
The BBC's diplomatic correspondent Jonathan Marcus says that Beijing's chief concern is the stability of North Korea and it does not want to open the way to anything that might look like a naval blockade of the country.
The Security Council... condemns the nuclear test proclaimed by the DPRK on 9 October 2006 in flagrant disregard of its relevant resolutions
Most computers will open PDF documents automatically, but you may need to download Adobe Acrobat Reader
US President George W Bush said the UN had taken a "swift and tough" step to show its determination to keep the Korean Peninsula free of nuclear weapons.
Japanese Prime Minister Shinzo Abe said his country was already considering further punitive measures against Pyongyang.
His government unilaterally imposed some sanctions on Wednesday, banning all North Korean imports and stopping its ships entering Japanese waters.
International talks are likely to intensify in the coming days as governments work out practical steps to implement sanctions.
Ms Rice is due to visit Japan, South Korea and China later in the week to discuss enforcement.
The UN will also need to set up ways of overseeing the functioning of the sanctions regime.
The South Korean foreign ministry said it would honour the new sanctions and urged the North to return to the six-nation talks.
Russian envoy Alexander Alexeyev, who held talks in Pyongyang before travelling on to Seoul, said the North Koreans had told him they were not rejecting the six-sided negotiations.
"The North Korean side repeatedly insisted that the six-sided process should continue," Interfax news agency quoted him as saying.
Aid organisations say they are concerned that the sanctions will hurt still further a country barely able to feed itself.
"It is a very fragile country and there is a lot of hardship and we are trying to take care of the people," Jaap Timmer, head of the Red Cross's Pyongyang delegation, told AFP news agency.
"Humanitarian aid should not be dependent on political decisions and so we are hoping that from a moral point of view, any pressure on the government will not impact on ordinary people."
Erica Kang of the South Korean humanitarian group Good Friends said that there were worrying signs of hunger reminiscent of the 1990s famines.
"There are more people eating alternative foods, having to forage rather than having grain for their main meal... winter is coming shortly and we are very concerned about that," she said.