China opposed unilateral sanctions levied on Iran by the European Union (EU), said Foreign Ministry spokeswoman Jiang Yu on Friday.

"We hope the relevant parties will adhere to diplomatic means on the issue," said Jiang in response to a question.

China maintains that the Iran nuclear issue should be resolved through dialogue and negotiations, she added.

EU foreign ministers on Monday approved tougher sanctions on Iran over its refusal to halt its uranium enrichment program, including measures to block oil and gas investment.

EU's restrictive measures, which went beyond UN sanctions imposed on June 9, followed a similar move by the U.S.

On July 1, U.S. President Barack Obama signed a new Iran sanction bill into law, which was described by him as "striking at the heart" of Iranian government's nuclear ability.

Iran has submitted a letter to the Director General of the International Atomic Energy Agency (IAEA) Yukiya Amano on resuming the nuclear talks.

The letter, jointly provided by Iran's National Security Supreme Council and Iran's Atomic Energy Organization, expresses Iran's readiness to start uranium swap talks, said Iran's local media on Monday.

"We welcome Iran's move on the issue," said Jiang.

China hoped the talks between Iran and the Vienna Group, comprising the United States, France, Russia and the IAEA, would start as soon as possible, she said.

"We hope the relevant parties will reach consensus on the issue at an early date," said Jiang, adding this will help promote the process of resolving the Iran nuclear issue through dialogue and negotiations.