"As I have previously said, we cannot trust in the words of the United States and even in their signature, so negotiations with the United States are useless," Khamenei told a gathering of Iranian diplomats in Tehran.

"The idea that problems can be resolved through negotiations or relations with the United States is a major error," he added.

Having withdrawn from the landmark 2015 nuclear deal between Tehran and world powers, Washington is determined to isolate Iran and pile on economic pressure with a full reimposition of sanctions, starting in August.

Europe opposes the move and has vowed to find ways of maintaining its trade ties with Iran, which under the deal curbed its nuclear programme in exchange for a lifting of sanctions.

"Negotiations with the Europeans must continue, but we must not wait for their offer indefinitely," said Khamenei.

President Donald Trump has said he is open to a new deal that would cover not only Iran's nuclear facilities, but also its missile programme and regional interventions which are seen as a threat to Washington's Israeli ally.

"The US seeks the return of the situation and their status before the (1979 Islamic) revolution in Iran," said Khamenei.

"They are against the nuclear potential and the power of its (Iran's) enrichment, and its presence in the region."

Iran was a close ally of the United States up until the revolution.