WASHINGTON, Oct. 23 -- U.S. chief nuclear negotiator Thursday called for a "wise" use of the remaining time to pursue a comprehensive deal over Iran's nuclear program by Nov. 24.

Wendy Sherman, under secretary of state for political affairs, said she does not know whether the deal can be reached by the cut-off date.

"Despite intense efforts of the negotiators, we are still in that 'difficult' stage," she said in remarks delivered at the Center for Strategic and International Studies, a Washington-based think tank.

"We must use the remaining time wisely and with a sense of urgency and purpose," she added.

Washington has said its focus is on the ongoing negotiations with Iran rather than on extending the deadline once again. Iran and the so-called P5+1 group -- Britain, China, France, Russia, the United States plus Germany -- agreed in July to extend the talks for another four months till Nov. 24, as they could not narrow down significant gaps on core issues during the past six months.

The two sides met again last week for three days in Vienna, Austria without any major breakthrough.

Sherman said "impressive progress" has been made during past talks on issues "that originally seemed intractable," but differences remain with the size and scope of Iran's uranium enrichment capacity drawing more attention.

"If Iran truly wants to resolve its differences with the international community and facilitate the lifting of economic sanctions, it will have no better chance than between now and Nov. 24," she said. "This is the time to finish the job."

"The United States and its partners are prepared to take advantage of this historic opportunity to resolve our concerns regarding Iran's nuclear program," she remarked.