SINGAPORE — Relations between the United States and China — already under serious strain over trade and economic issues, human rights and, most recently, the Yellow Sea and the South China Sea — are about to be tested anew over Iran's controversial nuclear program.

Armed with what it says are extended international and national sanctions to squeeze Iran's banking, foreign trade, energy, and air and sea transport sectors, the U.S. is sending senior officials to Asia, the Middle East and South America this month to try to rally more government and corporate support.

The appeal is backed by a threat: costly penalties for companies, whether state-owned or private, that defy the sanctions. The penalties could include being banned from operations in the U.S., barred from access to the U.S. financial system and loss of U.S. government contracts.