Iran has sent a "constructive" response to U.S. proposals aimed at reviving Tehran's 2015 nuclear deal with world powers, Iranian Foreign Ministry spokesperson Nasser Kanaani was quoted by state media as saying on Friday.

"The text that was sent (by Iran) has a constructive approach aimed at finalizing the negotiations," Kanaani was quoted as saying by state broadcaster IRIB, without giving further details.

But the U.S. State Department gave a different assessment.

"We can confirm that we have received Iran's response through the EU," a spokesperson said. "We are studying it and will respond through the EU, but unfortunately it is not constructive."

The IRIB report said Iran's response was sent to EU foreign policy chief Josep Borrell, who has been coordinating the negotiations. It gave no further details.

After 16 months of indirect talks between Tehran and Washington, Borrell said on August 8 the EU had laid down a final offer to overcome an impasse for the revival of the agreement.

Iran needs stronger guarantees from Washington for the revival of a 2015 nuclear deal, its foreign minister said on Wednesday, adding that the UN atomic watchdog should drop its "politically motivated probes" of Tehran's nuclear work.

Under the 2015 pact, Iran had curbed its nuclear program in return for relief from U.S., EU and UN sanctions.

On Friday, the Chinese Foreign Ministry responded that the Chinese side is carefully examining Iran's feedback.

Initially, the feedback from the Iranian side does not go beyond its consistent position on relevant issues, spokesperson Zhao Lijian noted at a regular press.

"We hope that all parties, especially the U.S. side, will respond positively to the reasonable concerns of the Iranian side and promote the early conclusion of an agreement," Zhao said, adding China is ready to continue to play a positive role for that to be realized.

(With input from Reuters)