Washington, DC – A group of Democrats in the United States Congress have called on the State Department to break the US government’s longstanding silence on Israel’s nuclear capabilities.

In a letter sent to Secretary of State Marco Rubio, the Democrats pointed to the US-Israel war on Iran as the reason more clarity is urgently needed.

While Israel is believed to have possessed nuclear weapons since the 1960s, it maintains “a policy of nuclear opacity, never officially confirming the existence of its nuclear weapons program and arsenal”, according to the Washington, DC-based Nuclear Threat Initiative.

The White House has also long maintained ambiguity on the issue, despite a handful of glancing admissions. In turn, lawmakers in Congress have launched several coordinated public efforts for greater transparency amid decades of bipartisan support for Israel.

“Congress has a constitutional responsibility to be fully informed about the nuclear balance in the Middle East, the risk of escalation by any party to this conflict, and the administration’s planning and contingencies for such scenarios,” the letter, signed by 30 members of Congress, said. “We do not believe we have received that information.”

“A policy of official ambiguity about the nuclear capabilities of one party to this conflict makes coherent nonproliferation policy in the Middle East impossible,” the letter said, “for Iran, for Saudi Arabia, and for every other state in the region making decisions based on their perceptions of the capabilities of their neighbours.”

What does the letter ask for?

In the letter, dated May 4, the lawmakers plainly ask Rubio what nuclear weapon capabilities Israel has, as well as clear information on its warheads and launchers.

They particularly focused on the Negev Nuclear Research Center in Dimona, long believed to be the core of Israel’s nuclear programme.

“Does Israel currently possess enrichment capabilities, and at what level?” they asked, appealing for details on both fissile material and plutonium production.

The letter further asked whether Israel, which is not a signatory to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), has articulated to the US any “nuclear doctrine, red lines, or thresholds for nuclear use in the context of the current conflict with Iran?”

“Has the administration received any assurances from Israel that nuclear weapons will not be used?

“Have there been any indications of Israel planning to use or deploy nuclear weapons during the recent Iran conflict or during other conflicts?”

What is known about Israel’s nuclear programme?

Several former US officials, Israeli whistleblowers, and unclassified US intelligence documents have for decades shed light on Israel’s alleged nuclear programme.

Documents show that in 1968, the CIA told then-US President Lyndon B Johnson that Israel had developed or was capable of developing a nuclear weapon. President Richard Nixon then reportedly struck an agreement with Israeli Prime Minister Golda Meir in which Israel agreed not to acknowledge or test its nuclear arsenal in exchange for Washington ending oversight pressures.

Israeli nuclear technician-turned-whistleblower Mordechai Vanunu leaked evidence of the Negev Nuclear Research Centre to the United Kingdom’s Sunday Times in a landmark 1968 report.

In its letter to Rubio, US lawmakers noted that the “public record strongly and consistently supports the conclusion that Israel possesses nuclear weapons”. It pointed to a 1974 US Special National Intelligence Estimate and several statements by US and Israeli officials.

US officials included former defence secretary nominee Robert Gates, who, during testimony to the Senate Armed Services Committee in 2006, listed Israel as one of the world’s “powers with nuclear weapons”.

The Nuclear Threat Initiative estimates that Israel has 90 nuclear warheads, a plutonium stockpile of 750 to 1,110kg (1,700 to 2,400lbs), six submarines capable of launching nuclear weapons, and intermediate-range ballistic missiles capable of reaching 4,800 to 6,500km (3,000 to 4,000 miles).

What is the significance of the letter?

Individual lawmakers have previously called for more transparency on Israel’s nuclear weapons. For example, Representative James McGovern referenced Israel as a nuclear-armed nation in a resolution in 2019.

Still, concerted congressional pressure on US presidential administrations has been exceedingly rare.

The letter comes as lawmakers from both parties have increasingly questioned Washington’s close ties to Israel amid the genocide in Gaza and the US-Israel war on Iran.

In April, 40 Democratic senators voted in support of a bill to block the sale of military bulldozers to Israel. While the measure failed, advocates hailed the increased pressure among Democrats as “historic”.

The Trump administration, meanwhile, has said that preventing Iran from developing a nuclear weapon is a key objective in its war. Tehran has for years denied seeking such a weapon.

In a statement to Al Jazeera, Josh Reubner, the policy director for the Institute for Middle East Understanding Policy Project, hailed lawmakers for their calls for clarity on Israel’s nuclear weapons programme.

“This initiative is taking place against the backdrop of the US-Israeli war of aggression against Iran. One of Trump’s goals for ending this war involves negotiations to lift sanctions against Iran in exchange for an Iranian commitment not to develop nuclear weapons,” Reubner said.

“Members of Congress are right to question why Israel’s development of nuclear weapons gets a free pass while we’re trying to prevent Iran from acquiring them,” he said.