Ali Shamkhani, the Secretary of Iran's Supreme National Security Council, stated that the nuclear discussions are growing "knottier by the hour" due to the "absent US political decision."

"US approach to Iran's principled demands, coupled with its unreasonable offers and unjustified pressure to hastily reach an agreement, show that US isn't interested in a strong deal that would satisfy both parties," he wrote on Twitter.

Shamkhani's announcement came hours after Iran's senior negotiator, Ali Bagheri, returned to Austria's capital, Vienna, to conclude negotiations with the United States.

Tehran is attempting to make significant progress by resolving the issue of freely selling oil, entering crude money in foreign currency into Iranian bank accounts, and enjoying the economic perks outlined in the nuclear deal.

The US withdrew from the international nuclear agreement made in Vienna in 2015 to restrict Iran's nuclear program in exchange for the easing of international sanctions. It then reimposed sanctions on Tehran, which had a significant economic impact on the Islamic Republic.