More than 30 Democrats in the US House of Representatives have penned a letter to Secretary of State Marco Rubio demanding that the Trump administration publicly acknowledge Israel's clandestine nuclear weapons programme.

Israel has never formally declared the programme's existence, and US officials have spent decades maintaining a purposeful ambiguity on it. But the congressional Democrats, led by Joaquin Castro, are demanding that era come to an end amid the stalled US-Israeli war with Iran.

"We are, in the fullest sense, fighting this war side by side with a country whose potential nuclear weapons program the United States government officially refuses to acknowledge," the lawmakers wrote on Tuesday in a letter to Rubio.

The lawmakers said that the US acknowledges the nuclear capabilities of many countries, friend or foe, including Russia, China, Pakistan, India, France and North Korea.

"The risks of miscalculation, escalation, and nuclear use in this environment are not theoretical," they said.

"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. We do not believe we have received that information."

The lawmakers noted that Iran targeted Israel's nuclear facilities in the southern city of Dimona during this year's war, asking Rubio questions about the site, including whether it produces fissile material for a bomb, including plutonium.

"The public record strongly and consistently supports the conclusion that Israel possesses nuclear weapons," they wrote.

The Democrats cited public information disclosed by Mordechai Vanunu, a technician at the Negev Nuclear Research Centre, who presented evidence of Israel's nuclear weapons programme to the British media in 1986, as well as a US intelligence assessment from 1974 that was not made public until 2008, as evidence that the programme exists.

They also pointed to testimony from then-Defence Secretary nominee Robert Gates, who confirmed during his confirmation hearing that Israel has nuclear weapons while discussing Iran's ambitions.

"They are surrounded by powers with nuclear weapons — Pakistan to their east, the Russians to the north, the Israelis to the west and us in the Persian Gulf," he said.

The lawmakers asked Rubio to respond by May 18.

Blocked Negotiations

Israel remains the only country in the world that has not confirmed possession of such weapons.

In 2017, it blocked negotiations on the Treaty on the Prohibition of Nuclear Weapons and has consistently voted against UN General Assembly resolutions on the treaty since then.

While it is hard to pinpoint when exactly Israel started its nuclear programme, it likely developed it shortly before the Six-Day War in 1967 against Egypt, Syria and Jordan.

Since the early 1990s, Prime Minister Benjamin Netanyahu has claimed that Iran was close to acquiring nuclear weapons.

Iran, on the other hand, has long denied seeking nuclear weapons, insisting its nuclear programme is for civil use.