"The notion that we have delayed this negotiation in any way is just not true," said State Department spokesman Ned Price.

After the EU sent the proposed text to both Tehran and Washington in late July, Iran "responded with several comments," Price said, without being specific.

"This is why it has taken us some additional time to review those comments and to determine our response of our own," he said.

"We are seriously reviewing those comments."

Earlier Monday, EU foreign policy chief Josep Borrell, who is leading the effort to bring the United States and Iran together in agreement, suggested Washington was now slowing the process.

"There was an Iranian response that I considered reasonable to transmit to the United States," he said.

"The United States has not formally replied yet. But we are waiting for their response and I hope that response will allow us to finish the negotiation -- I hope so, but I can't assure you of it."

Price said Washington was "encouraged" by the fact that Tehran appeared to have dropped an earlier demand that, to complete a deal, the United States remove its formal designation of the Islamic Republican Guard Corps as an international terrorist organization.

That was one of the issues that has appeared to hold up progress on a final agreement that was sketched out in March.

However, Price added, "there are still some outstanding issues that must be resolved, some gaps that must be bridged, if we are able to get there."

"We are working as quickly as we can to put together an appropriate response to the Iranian paper," he said.

Iran and major global powers struck a deal in 2015 to limit Tehran's nuclear program with the aim of preventing it from obtaining nuclear weapons.

But in 2018 US President Donald Trump, a strong critic of the so-called Joint Comprehensive Plan of Action (JCPOA), unilaterally pulled out and slapped heavier sanctions on Iran.

Since then Iran has accelerated its nuclear research and development activities, getting closer to where it would be able to create a nuclear bomb.

Since coming into office in January 2021, US President Joe Biden has pressed to revive the JCPOA in exchange for alleviating sanctions on Iran.