Saudi Arabia appears to be using US-supplied white phosphorous munitions in its war in Yemen, based on images and videos posted to social media, raising concerns among human rights groups that the highly incendiary material could be used against civilians.

Under American regulations, white phosphorous sold to other countries is to be used only for signaling to other troops and creating smoke screens. When the munition explodes, it releases white phosphorous that automatically ignites in the air and creates a thick white smoke. When used against soldiers or civilians, it can maim and kill by burning to the bone.

It is unclear exactly how the Saudis are using the munitions, but the government has already received widespread condemnation for its indiscriminate bombing in civilian areas since its campaign against rebel forces in Yemen began in 2015.

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US officials confirmed that the American government has supplied the Saudis white phosphorous in the past but declined to say how much had been transferred or when. After reviewing a social media image taken from the battlefield that showed a white phosphorous mortar shell, a U.S. official said it appeared to be American in origin but could not trace it to a particular sale because some of the markings were obscured.

"The United States expects any recipient of US military assistance to use those items in accordance with international law and under the terms and conditions of any US transfer or sale," said a State Department official, who spoke on the condition of anonymity to discuss politically sensitive issues.

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The official said the department was looking into reports of Saudi forces' improperly using US-supplied white phosphorous munitions. "If a country is determined to have used U.S.-provided weapons for unauthorised purposes, the US will take appropriate corrective action," the official said.

The United States has grown increasingly wary of its material support to the Saudi military. In May, the Obama administration halted the sale of roughly 400 cluster bombs to the Saudis after human rights organizations documented the weapons' use in civilian areas. This week, lawmakers on Capitol Hill moved to delay a $1 billion arms deal that would replace some of Saudi Arabia's US-supplied tanks that have been damaged in the conflict.

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Since coming to office in 2009, the Obama administration has facilitated more than $115 billion in 42 different arms sales to Saudi Arabia, more than any other US administration, according to a report in the Security Assistance Monitor. Batches of the equipment are likely to be delivered for years to come.

International humanitarian law does not ban the use of white phosphorous outright, but there is a strict requirement that it be used only in areas clearly separated from civilians. Even using it against enemy combatants has raised concerns, given that the munitions can cause particularly horrific injuries.

"The United States must not provide or sell white phosphorous munitions to Saudi Arabia or any other military that would use them in the Yemen conflict," said Sunjeev Bery, Amnesty International's advocacy director for the Middle East and North Africa. "As a major arms seller to Saudi Arabia, the U.S. risks being complicit in Saudi Arabia's likely war crimes in Yemen."

A spokesman from the Saudi Embassy in Washington did not respond to requests for comment for this article. The Saudi government has repeatedly denied claims about unlawful bombings and civilian casualties, pointing to its military's Western support as validation of its practices.

According to the United Nations Office of the High Commissioner for Human Rights, more than 3,700 civilians have been killed and 2.8 million have been displaced during the now nearly two-year-old war.

"The United States is concerned by the high number of casualties resulting from this war," the State Department official said. "We are prepared to work with the Saudis to deter and confront any external threat to their territorial integrity, and we stand by that assurance. However, that does not mean we refrain from expressing our concerns about the war in Yemen and how it has been waged."

The Pentagon provides midair refueling for Saudi aircraft and limited intelligence resources to Saudi forces. In addition to short-term military assistance, the Pentagon and the State Department, as well as other Western countries, have facilitated the sale of billions of dollars worth of arms to the Kingdom, everything from hand grenades to attack helicopters.

A Pentagon spokesman, Navy Cmdr. Patrick Evans, said that "as a matter of policy," the Pentagon office responsible for overseeing foreign military sales "does not disclose specific deliverables or the details" of the final transfer agreements.

The United States has used white phosphorous against fighters, including in 2004 in Fallujah, Iraq, and sporadically in Afghanistan over the course of the war there. In 2009, Israel used the weapon in populated areas in the Gaza Strip.

Images on pro-Saudi Twitter and Instagram accounts show that Saudi forces are using several systems for firing white phosphorous munitions, including tank rounds, mortars, howitzers and rifle grenades.

Footage and images and social media posts showing the munition indicate that it is being used near the Saudi-Yemen border -- in Najran province -- and around the Yemeni capital, Sanaa.

The most recent footage -- posted Sept. 9 -- shows a U.S.-developed M198 155mm Saudi howitzer with the telltale sea-foam green white phosphorous rounds nearby ready to be loaded and fired.

Many of the images posted to social media show white phosphorous rounds from a distance, leaving any writing or identifying features blurry or ill-defined. The shell's color pattern is the most telling, as the greenish-hue interrupted by yellow bands and red writing is internationally recognized as indicating white phosphorous munitions.

The picture reviewed by the U.S. official was first posted in November 2015 on a Saudi Instagram account and shows the shell with the words "Martyr Jamil Hadi" written on it.

The only company with the rights to sell to the U.S. government the white phosphorous round pictured in the image is General Dynamics Ordnance Tactical Systems, according to Marine Corps Systems Command documents distributed in 2015. The shell's design is owned by TDA-Armaments of France. Both TDA-Armaments and General Dynamics manufacture the munition, although it is unclear which country manufactured the round pictured.

When asked about the image, General Dynamics spokeswoman Laurie VonBrocklin said "it wouldn't be appropriate for us to comment" and deferred further questions to the Pentagon and the State Department.