Iran's Supreme Leader Ayatollah Ali Khamenei is reportedly imposing a ban on further negotiations with the United States, putting brakes on moderates hoping to end Iran's isolation after reaching a nuclear deal with world powers in July.

In an address to Revolutionary Guard Navy commanders last week, Khamenei stressed that negotiations with the US provide more disadvantages than benefits for his country.

"Negotiation with the US is forbidden because of the numerous disadvantages that it has and the benefits that it doesn't have. This is different to negotiating with a state which has no such facilities and no such motivation (against Iran),'' Ali-Khamenei said, according to Reuters.

"Through negotiations Americans seek to influence Iran ... but there are naive people in Iran who don't understand this," he was quoted saying.

Khamenei, the highest authority in the Islamic Republic, has reportedly supported the last 18 months of negotiations but has not publicly endorsed the nuclear agreement with the United States, Germany, France, Britain, China and Russia.

The agreement seeks to curb Tehran's nuclear programme in exchange for lifting crippling Western sanctions on Iran. It was reportedly welcomed by Iranians, including Iran President Hassan Rouhani, who are keen to improve Iran's living standards as well promote better ties with the rest of the world.

Rouhani said his government is ready to hold talks with the United States on how to resolve the conflict in Syria, where both countries back opposing sides.

After signing the deal last July, Rouhani was quoted saying that Tehran should no longer be regarded as an ''international threat.''

Meanwhile, hundreds of Iranian troops arrived in Syria last month to join government forces and their Lebanese Hezbollah allies in a major ground offensive backed by Russian air strikes, according to Reuters.

Khamenei warned the Guards that Iran is facing threats from enemies who are "trying to change the mentality of our officials and our people on the revolution and our national interests.''