The United States has reached out to China diplomatically about reducing its purchases of Iranian crude oil, US and European officials said on Tuesday, as Washington seeks to persuade Tehran to resume talks about reviving the 2015 nuclear deal.

Purchases of Iranian oil by Chinese companies are believed to have helped keep Iran's economy afloat despite US sanctions that are designed to choke off such sales to put pressure on Iran to curb its nuclear program.

"We are aware of...