China said on Tuesday it would safeguard its interests after Washington laid out plans for the "economic asphyxiation" of Beijing's economic partner Iran.
Almost six months into its war with Iran, the United States issued new penalties on some entities including in China and expanded secondary sanctions threats, urging governments worldwide to join its economic campaign.
China has advocated for a ceasefire, and insisted US sanctions will not resolve the conflict.
"Cooperation between China and Iran has always been conducted within the framework of international law and should not be interfered with or disrupted," foreign ministry spokesman Lin Jian told a regular news briefing on Tuesday.
"China has already stated many times that it firmly opposes illegal unilateral sanctions... China will take all necessary measures to firmly safeguard its own rights and interests," he added.
Beijing is a key customer for Iranian oil, and is directly affected by the blockage of the Strait of Hormuz.
Iran has weathered sanctions for decades, using complex international financial networks to evade restrictions. Before the war, it continued to export millions of barrels of oil, mostly to China.
Asked on Monday if Chinese banks would be targeted by the new penalties, US Treasury Secretary Scott Bessent said "no one is above the reach of US sanctions".