"The written proposal we received from the US contains many ambiguities and questions. Many issues in this proposal are not clear," Araghchi said while on a visit to Lebanon.

On Saturday, Iran said it received "elements" of a US proposal for a nuclear deal following five rounds of talks that started in April and were mediated by the sultanate of Oman.

Uranium enrichment has remained a key point of contention between Washington and Tehran in the talks to seal a nuclear deal, with Iran defending what it says is its right to pursue a peaceful nuclear energy programme and the US calling it a "red line".

US President Donald Trump on Monday reiterated that Iran will not be allowed any enrichment of uranium under a potential deal with Washington.

"Continuing enrichment on Iranian soil is our red line," Iran's top diplomat said on Tuesday.

He added that his country would submit a response to the proposal in the coming days, based on Iran's "principled positions and the interests of the Iranian people".

"We will not ask anyone for permission to continue enriching uranium in Iran. However, we are ready to take steps... to ensure that this enrichment will not lead to the production of nuclear weapons," he said.

Araghchi was visiting Beirut following a stop in Cairo on Monday, where he met with Rafael Grossi, the head of the International Atomic Energy Agency (IAEA).

The head of the UN nuclear watchdog on Monday called for more transparency from Iran following a leaked report that showed Tehran had increased production of highly enriched uranium.

The IAEA report showed that Iran has ramped up production of uranium enriched up to 60 percent purity -- close to the roughly 90 percent level needed for atomic weapons.

Western countries, including the United States, have long accused Iran of seeking to acquire nuclear weapons, while Iran insists its nuclear programme is for peaceful purposes.