The agreement sealed Saturday has triggered commentary among US conservatives of a new anti-US "axis" and would have likely been sharply criticized by former president Donald Trump's administration.

But President Joe Biden's government declined to wade in on the pact, which has also provoked opposition inside Iran.

"Competition, as you know, does define our relationship with China, but we do have, in some cases, rather narrow areas of tactical alignment," State Department spokesman Ned Price told reporters.

"It so happens that Iran is one of them. China has been cooperative in efforts to constrain Iran's nuclear program," he said.

"Beijing, of course, has no interest in seeing Iran develop a nuclear weapon, and the profoundly destabilizing impact that would have in a region upon which China does depend."

China was one of six powers that was part of the 2015 Joint Comprehensive Plan of Action, under which Iran agreed to curtail its nuclear program in return for sanctions relief.

Trump trashed the deal but Biden has hoped to revive it, although he insists Iran must return to full compliance before he ends Trump's sanctions.

Price said China and the United States had common interests "as we look to ways to find that mutual return to compliance with the JCPOA."

China has remained the top, if diminished, buyer of Iranian oil since Trump imposed unilateral US sanctions in 2018 to warn all countries against buying from the clerical state.

Even if Biden lifts sanctions, China would remain the privileged buyer of Iranian oil under the pact. Japan, South Korea and India, all aligned with the United States, mostly complied with Trump's sanctions.

China signed the deal with Iran on a regional trip by Foreign Minister Wang Yi, who also visited Turkey and Saudi Arabia, allies of the United States.