China has opposed sanctions in principle, favoring instead what Beijing calls "negotiations and diplomacy."
U.S. officials are uncertain what is behind the recent cancellation of Mr. Einhorn's visit but are concerned it is a sign that Beijing is having second thoughts about supporting U.S.-led diplomatic efforts for international pressure against the two states.
The United States imposed new sanctions against North Korea in August, targeting intelligence units and those trafficking in luxury goods.
The United Nations in June imposed its fourth round of sanctions against Iran to punish Tehran for its nuclear program. The U.N. resolution sought to bolster existing sanctions aimed at the economic, high-technology and military sectors. It calls for U.N. member states to chase Iranian ships suspected of carrying banned goods and also tries to increase pressure on banks and insurance firms to cut ties with Iran.
U.N. sanctions were imposed on North Korea in 2009 after the North's second underground nuclear test. Those sanctions included the inspection of North Korean ships, a wider ban on arms sales and other financial measures.
U.S. officials said China has failed to enforce sanctions fully on either country, although some steps were taken.
China is North Korea's main trading partner in fuel oil, and both countries' Communist parties and militaries maintain close relations.
China's opposition to Iran sanctions is based on extensive Chinese trade and economic dealings with Iran, specifically purchases of Iranian oil.
Chinese President Hu Jintao told President Obama in April that China would support sanctions on Iran.
McChrystal probe
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