Secretary of State John Kerry gestures during a news conference after he delivered remarks to the United Nations Human Rights Council on Monday, March 2, in Geneva. (Evan Vucci/AP)

Secretary of State John F. Kerry and senior Iranian envoys moved ahead Tuesday with talks over Tehran’s nuclear program, suggesting some progress was made even as Israeli’s prime minister prepared to denounce a possible deal in Washington.

Kerry said the biggest hurdle now is whether Tehran is willing to make tough political decisions to strike a deal before a self-imposed deadline a few weeks away despite hard-line opposition at home.

Iran’s foreign minister, Mohammad Javad Zarif, noted that the talks were “able to make progress.”

But he also seemed to indicate that serious difference remain. Iran’s Fars news agency quoted Zarif as rejecting as “unacceptable” a proposal outlined by President Obama to effectively freeze Iranian nuclear development for at least a decade.

“Iran will not accept excessive and illogical demands,” Fars quoted Zarif as saying.

Iran's potential nuclear capability
Graphic: Iran's potential nuclear capability

(BONNIE BERKOWITZ AND RICHARD JOHNSON/THE WASHINGTON POST/SOURCE: The Wisconsin Project on Nuclear Arms Control's;International Atomic Energy Agency; Institute for Science and International Security)

Nevertheless, Zarif said Iran would continue negotiating.

[Here’s your CliffsNotes for the Netanyahu speech]

“There is a seriousness that we need to move forward,” Zarif said at the Swiss resort of Montreux, according to the Associated Press.

Neither side has given details of the direction of the talks, which seek to find ways to limit and monitor Iran’s nuclear program. On Monday, however, Obama said one key proposal aims for least a 10-year freeze on any advances in Iran’s nuclear capabilities, and possible roll backs in some areas.

The West and its allies worry that Iran’s uranium enrichment labs could eventually produce material for nuclear weapons. Iran insists it does not seek nuclear arms, but wants the ability to make nuclear fuel for energy-producing reactors and medical research.

But Netanyahu views any nuclear self-sufficiency by Iran as a threat to Israel’s “survival.” His speech to a joint meeting of Congress on Tuesday (10:45 a.m. EST) is expected to issue a direct attack on the talks and appeal to U.S. lawmakers to rally behind Israel.

It’s unlikely to derail to current push in the negotiations. Obama, who does not plan to meet with Netanyahu, called the Israeli leader’s address to Congress a “distraction” from the overall goal of finding ways to control Iran’s nuclear technology.

Jason Rezaian’s journey has taken him from a childhood in San Francisco to his father’s native Iran. At 37, he became the Washington Post correspondent in Tehran. In July 2014, he was thrown into Iran’s Evin Prison, where he remains, without access to a lawyer. This is his story. (Jorge Ribas/The Washington Post)

“I think that it is a distraction from what should be our focus. And our focus should be, ‘How do we stop Iran from getting a nuclear weapon?’” Obama said Monday in an interview with the Reuters news agency.

In Montreux, Kerry was joined by Energy Secretary Ernest Moniz. The Iranian side included Iranian nuclear chief Ali Akbar Salehi and Hossein Fereidoun, the brother and aide to President Hassan Rouhani.

Their participation is an indication that the negotiations have reached a highly technical stage, involving complex details of the types of nuclear technology that Iran would be allowed to operate.

In related diplomacy, Kerry plans to visit Saudi Arabia later this week to brief King Salman on the talks. Saudi backing for a potential deal is critical to calm fears among the Western-allied Gulf Arab state, which view Iran as a major regional rival.

Brian Murphy in Washington contributed to this report.