A former senior US official on Thursday used tough rhetoric in warning the Obama administration to employ vigilance in engaging Iran in the ongoing nuclear negotiations with world powers.

“The Iranians are known as great rug merchants, not for nothing,” the 92-year-old former US secretary of state George Shultz told the BBC in an interview. “They’re good at this business of smiling, encouraging you on and then cutting your throat, so you have to be tough-minded, you have to be realistic.”

Shultz added that Iran is “a pretty tough customer” and the world’s largest state-supporter of terrorism, including support for Hezbollah in Lebanon.

The French foreign minister on Thursday said world powers needed to be firm in their dealings with Iran over the country’s nuclear program during talks in Geneva, which began on Wednesday.

Asked by France 2 television whether he thought there could be a deal with Tehran this week, Laurent Fabius said, “I hope so.”

“But this agreement can only be based on firmness,” he added.

Iran and six world powers — the US, the UK, France, Russia, China and Germany — reconvened talks Thursday morning, and Iran’s Deputy Foreign Minister Abbas Araqchi told the press tha Tehran wouldn’t accept any plans which bar Iran’s right to enrich uranium, reiterating Ayatollah Ali Khamenei’s statement that such a demand is a “red line.” He added that the Islamic Republic wouldn’t take big steps for small concessions, but voiced optimism that talks would continue.

A brief session of talks between the P5+1 states and Iran ended Wednesday night with both sides expressing hope of a possible deal being signed Thursday.

US Secretary of State John Kerry said Wednesday that any agreement with Iran would not allow it to buy time, but added that an interim deal would not include a long-term decision on limiting the country’s nuclear enrichment.

Speaking to reporters in Washington, Kerry said an initial deal with Iran would not give Tehran a chance to rush to nuclear weapons capability.

“We will not allow this agreement, should it be reached … to buy time or to allow for the acceptance of an agreement that does not properly address our core, fundamental concerns,” Kerry said.

Iran is pushing for sanctions relief in exchange for curbing some of its nuclear activity. Reports indicate that an interim agreement would dial back some financial restrictions on Iran in return for Tehran keeping uranium below 20 percent enrichment.

Shultz, the former US secretary of state, also published an op-ed in the Wall Street Journal on Thursday in which he cautioned the US against rushing into an agreement with Tehran. “The guy who is anxious for a deal will get his head handed to him,” said the Reagan-era diplomat, who negotiated nuclear nonproliferation with the Soviets.

The former secretary of state pointed to Iran’s “increasing nuclear capacity and its unacceptable behavior” as points of comparison to nuclear negotiations with the Soviets.

“The election of President Hassan Rouhani, a ‘moderate’ in the eyes of some, may provide a slight opening. But don’t bet on it. At this point, strength in the form of sanctions is taking its toll. As with the INF negotiations [with the Soviets], the US shouldn’t be afraid to up the ante,” Shultz said.

“Moreover, if Iran has no intention of producing nuclear weapons, then Tehran should cease all uranium enrichment and immediately allow international inspections for verification,” he wrote. “Nuclear materials for power and research facilities are readily available and have been offered to Iran for such purposes for years.”

Iranian Foreign Minister Mohammad Javad Zarif also took to the editorial pages with Arabic and English op-eds in the London-based daily Al-Sharq Al-Awsat continuing Tehran’s so-called charm offensive against the West. Zarif called for regional amity and warns against outside actors — a veiled reference to the United States — whose “paramount interest… may not always be stability, but in fact may depend on what can justify their presence” in the Middle East.

“Still, I wish to reiterate that the Islamic Republic of Iran does not have any such illusions” of employing hostility to advance interests, Zarif wrote. “We recognize that we cannot promote our interests at the expense of others. This is particularly the case in relation to counterparts so close to us that their security and stability are intertwined with ours.”