"We reject the unilateral imposition of sanctions," said Fu Cong, Director General of the Chinese Foreign Ministry's Department of Arms Control.

The United States, which is trying to force Iran to renegotiate an agreement limiting its nuclear programme, at the end of May tightened sanctions to include a full ban on Iran's exports of oil and has threatened to punish nations which refuse to go along.

"For us energy security is important and the importation of oil is important to Chinese energy security and also to the livelihood of the people," said Cong.

The sanctions have led to a plunge in Iranian oil exports, a key source of revenue for the government.

But according to an International Energy Agency report earlier this month Iran was continuing to export some oil, although it was difficult to say to which countries the crude was being shipped.

"We do not accept this so-called zero policy of the United States," said Cong, who was speaking on the sidelines of a meeting on the implementation of the 2015 Iranian nuclear deal.

The deal with European powers, Russia and the United States relaxed trade restrictions on Iran in exchange for limits on its nuclear programme, but President Donald Trump has pulled Washington out of the agreement.

That has led to the unravelling of the deal that aimed to boost regional security.

Cong's statement comes a day before Trump is to meet his Chinese counterpart Xi Jinping on the sidelines of the G20 summit in Japan in an attempt to resolve an escalating trade war.