China has demanded that the US stop interfering in its trade affairs with Iran, after Washington raised concerns over a planned energy deal with Tehran.
China is looking to expand its energy investments abroad
Beijing said plans by a Chinese oil firm to invest in an Iranian gas field were "legitimate", news agency AP said.
The US government last month expressed concern over Chinese state-run oil firm CNOOC's investment proposals in Iran.
The US said the plan was inappropriate at a time when Iran is defying UN resolutions over its nuclear programme.
The proposed Chinese investment in Iran's Pars gas field could be worth $16bn (£8.2bn), according to Iranian news agency Mehr.
CNOOC said the company was currently in talks with Iranian officials to develop the northern gas field, although no agreement had yet been signed.
"We think this kind of co-operation and relationship is legitimate. Normal co-operation should not be interfered (with)," said Chinese foreign ministry spokesman Liu Jianchao.
The Associated Press said that when asked if Beijing believed the US was meddling in its business affairs with Iran, Mr Liu responded "this is our position".
China, which is one of the five permanent members of the UN Security Council, voted last month to impose sanctions on Iran for defying calls by the international community to cease uranium enrichment - a process which could produce weapons grade material for nuclear bombs.
Tehran has denied that it plans to develop atomic weapons, and insists it has a right to enrich uranium for civilian use in nuclear power stations.