The Biden administration’s new $500m military contract with Saudi Arabia contradicts the spirit of the White House’s public policy to bar all “offensive” weapons sales to the kingdom for use against the Houthis in Yemen, critics of the deal have alleged.

The military contract will allow Saudi Arabia to maintain its fleet of attack helicopters despite their previous use in operations in Yemen.

The administration’s decision to end so-called “offensive” weapons to Saudi Arabia was one of Joe Biden’s first foreign policy objectives, and reflected what the US president called his commitment to “ending all support” for a war that had created “a humanitarian and strategic catastrophe”.

Saudi Arabia was given permission by the state department to enter a contract to support the Royal Saudi Land Forces Aviation Command’s fleet of Apache helicopters, Blackhawks, and a future fleet of Chinook helicopters. It includes training and the service of 350 US contractors for the next two years, as well as two US government staff. The deal was first announced in September.

“To my mind, this is a direct contradiction to the administration’s policy. This equipment can absolutely be used in offensive operations, so I find this particularly troubling,” said Seth Binder, director of advocacy at the Project on Middle East Democracy.

The decision to approve the military maintenance contract comes as the Biden administration appears to be softening its approach to the kingdom, with several high-level meetings between senior administration officials and their Saudi counterparts.

Experts who study the conflict in Yemen and the use of weapons by Saudi Arabia and its allies say they believe that Apache attack helicopters have mostly been deployed along the Saudi-Yemen border. They also say that it is difficult to pinpoint specific violations of international humanitarian law that occurred as a result of the Saudis’ use of Apaches, in part because such detailed data is scarce and difficult to verify.

The Saudi-led coalition’s internal investigative body, known as the Joint Incidents Assessment Team (Jiat), absolves member governments of legal responsibility in the vast majority of incidents. Saudi Arabia, the UAE and Egypt are the only countries in the coalition with Apache fleets.

The most deadly violation of international humanitarian law involving documented use of an Apache occurred in March 2017, when 42 Somali refugees fleeing Yemen for Port Sudan, and one Yemeni civilian, were killed after their boat was hit by a missile from a coalition warship, then gunfire from an Apache helicopter.

A September 2017 report in AirForces Monthly magazine states that five Saudi-operated Apache helicopters had been lost in Yemen, which strongly suggested they had been used in offensive operations.

Tony Wilson, the founder and director of the Security Force Monitor project at Columbia Law School Human Rights Institute, said it was difficult to see how the military helicopter maintenance agreement would not support Saudi military operations in Yemen.

Michael Knights, a fellow at the Washington Institute for Near East Policy, said he believed that Apaches had been used in what he described as “defensive missions” along the Yemen border, and therefore the sale of the maintenance contract was not contrary to the White House’s public position. He said the move probably reflected the Biden administration’s acknowledgement that a Saudi defeat to the Houthis, who had received support from Iran, would send a “negative message”.

Asked whether the administration had reviewed the use of Apaches by Saudis before the contract was agreed, a state department spokesperson said that it had “closely reviewed all allegations of human rights abuses or violations of international humanitarian law”, including those associated with the Saudi-led coalition.

The department said it concluded that the “overwhelming majority” of incidents had been caused by air-to-ground munitions from fixed-wing aircraft, leading the administration to suspend two previously pending air-to-ground munitions deliveries.

The state department spokesperson said Biden had said since the early days of his presidency that the US would work with Saudi Arabia “to help strengthen its defenses, as necessitated by the increasing number of Houthi attacks into Saudi territory”.

“This proposed continuation of maintenance support services helps Saudi Arabia maintain self-defense capabilities to meet current and future threats. These policies are intertwined with the direction by President Biden to revitalise US diplomacy in support of the UN-led process to reach a political settlement and end the war in Yemen,” the spokesperson said.

But other experts said the $500m contract did represent a distinct shift by the White House, and was a sign that Biden has largely abandoned a campaign promise to turn the regime of Prince Mohammed into a “pariah”.

“Many experts will tell you that there is no differentiation between defensive and offensive weapons. And so I think that making this differentiation from the beginning was a purposeful attempt to create leeway to pursue military cooperation,” said Yasmine Farouk, a scholar at Carnegie Endowment for International Peace.

“When he first came to the White House they maintained his narrative about reviewing arms sales, until this sale happened,” Farouk added.

While the US is engaged in negotiations, Seth Binder said, its efforts have so far been unsuccessful. “They haven’t been able to change the dynamics on the ground or the calculus of the major players.”

Experts are also increasingly concerned about the lack of accountability for human rights violations after Bahrain, Russia, and other members of the UN human rights council voted to shut down the body’s war crimes investigations into Yemen.

The investigators have previously said that possible war crimes have been committed by all sides in the conflict.

One person close to the matter said it became clear about a week before the vote that the resolution extending the work of the so-called Group of Eminent Experts (GEE), as the investigators are known, was in trouble.

Bahrain, the person said, led the push against renewal, and a decision by Japan to abstain from the vote was ultimately “the thing that really killed it”, the person said.

“What this has done is sent a message that once again in the context of Yemen, Saudi and Gulf states have immunity and protection in terms of collective accountability for what’s happened in the last seven years,” the person said.

“Our job was to keep reminding parties of the war that you can’t just do this stuff without consequences. Now that voice is gone.”

A state department spokesperson said the US was deeply disappointed that the Human Rights Council did not renew the GEE mandate for Yemen.