WASHINGTON  Within hours of President Obama’s decision to penalize imports of automobile tires from China, Chinese officials accused the United States of violating international trade laws and succumbing to political pressure for protectionism.

“China is firmly opposed to this measure of serious commercial protectionism by the United States,” the Chinese ministry of commerce said Saturday.

The sharp and swift reaction, though not necessarily indicative of a serious new clash, amounted to a warning shot just at a time when American officials need China’s help on issues including the global economic recovery and possible additional sanctions on Iran over its nuclear program.

Few decisions by President Obama have appeared more uncomfortable and fraught with larger considerations than the blandly worded announcement at 9:15 p.m. Friday that he would impose a tariff of 35 percent on Chinese tires for cars and light trucks.