Secretary of State John F. Kerry joined high-level talks Sunday in Geneva that could curtail Iran’s nuclear program, as negotiators race to reach a comprehensive agreement by the end of next month.

Kerry met with his Iranian counterpart, Foreign Minister Mohammad Javad Zarif, in the lakeside city. In an indication that the talks are at a sensitive stage, they were joined by Ernest Moniz, the secretary of energy, and Iran’s nuclear chief, Ali Akbar Salehi. Hossein Fereydoon, the brother of Iranian President Hassan Rouhani and one of his close aides, also participated. Zarif told reporters that Rouhani’s brother came to ensure “better coordination with the president.”

Both Kerry and Zarif, however, sought to play down speculation that an agreement was imminent. Kerry said Saturday in London that significant gaps remain, a status update that has changed little in months. “There is still a distance to travel,” he said.

And Zarif told reporters Sunday that there are still differences in negotiating positions staked out in the talks, which he recently characterized as so spirited at times that his bodyguards, overhearing his raised voice, have poked their heads into the room to make sure nothing was wrong.

“The fundamental gap, in my view, is psychological,” Zarif said. “Some Western countries, the United States in particular, see sanctions as an asset, a lever to exert pressure on Iran. As long as this thinking persists, it will be very hard, difficult to reach a settlement.”

The United States and five other partners in the talks — Britain, France, Germany, China and Russia — are working to strike a deal in which Iran would agree to curb its ability to make nuclear weapons, something Iran says it has no interest in manufacturing. In exchange, international sanctions against Iran would be eased and could eventually be lifted.

The talks are taking place amid a heightened sense of urgency. Though a temporary agreement is in place through June, Kerry has said that if the parties cannot agree on the major principles of a long-term accord by late March, there’s no point in continuing. Deadlines for a pact have been extended twice, and Kerry said President Obama is unlikely to ask for a third extension.

In addition, the secretive talks are being held barely a week before Israeli Prime Minister Benjamin Netanyahu is scheduled to deliver a controversial speech to Congress that he says will lay out the case for why the Israeli government considers the deal with Iran very dangerous, for Israel and for the world.

Netanyahu has been highly critical of the Obama administration’s handling of the talks. On Sunday, he opened a cabinet meeting saying he was astonished that negotiations were continuing after a leaked report from the International Atomic Energy Agency reportedly said Iran was not fully cooperating with the agency’s monitors. Netanyahu said the deal could allow Iran “to develop the nuclear capabilities that threaten our existence.”

In Tehran, many hard-liners are opposed to making concessions on the nuclear program in talks with the West, particularly the United States.

“The Islamic Republic of Iran will not give up its nuclear rights,” Ali Akbar Velayati, a senior adviser to Supreme Leader Ayatollah Ali Khamenei, said Sunday.