Tehran, Feb 7 : An Iranian top security official has underlined cautious optimism about the progress in the Vienna talks on reviving the 2015 nuclear deal, calling for a "balance" in the parties' commitments. "Despite the limited progress in the Vienna talks, we are still far from achieving the necessary balance in the commitments of the parties," Ali Shamkhani, Secretary of Iran's Supreme National Security Council, tweeted on Sunday. "Political decisions in Washington are requirements for balance of commitments to reach a good agreement," he said. Iran and world powers signed a nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), in July 2015. However, former US President Donald Trump pulled Washington out of the agreement in May 2018 and reimposed unilateral sanctions on Iran. Since April 2021, eight rounds of talks have been held in the Austrian capital between Iran and the remaining JCPOA parties to revive the pact, Xinhua news agency reported. The US government announced on Friday that it was waiving sanctions on Iran's civilian nuclear program, which allows international nuclear cooperation projects. /IANS