WASHINGTON (Reuters) - The Obama administration went ahead with a $1.3 billion arms sale to Saudi Arabia last year despite warnings from some officials that the United States could be implicated in war crimes for supporting a Saudi-led air campaign in Yemen that has killed thousands of civilians, according to government documents and the accounts of current and former officials.

Firefighters try to extinuish fire at the community hall where Saudi-led warplanes struck a funeral in Sanaa, the capital of Yemen, October 9, 2016. REUTERS/Khaled Abdullah

State Department officials also were privately skeptical of the Saudi military’s ability to target Houthi militants without killing civilians and destroying “critical infrastructure” needed for Yemen to recover, according to the emails and other records obtained by Reuters and interviews with nearly a dozen officials with knowledge of those discussions.

U.S. government lawyers ultimately did not reach a conclusion on whether U.S. support for the campaign would make the United States a “co-belligerent” in the war under international law, four current and former officials said. That finding would have obligated Washington to investigate allegations of war crimes in Yemen and would have raised a legal risk that U.S. military personnel could be subject to prosecution, at least in theory.

For instance, one of the emails made a specific reference to a 2013 ruling from the war crimes trial of former Liberian president Charles Taylor that significantly widened the international legal definition of aiding and abetting such crimes.

The ruling found that “practical assistance, encouragement or moral support” is sufficient to determine liability for war crimes. Prosecutors do not have to prove a defendant participated in a specific crime, the U.N.-backed court found.

Ironically, the U.S. government already had submitted the Taylor ruling to a military commission at Guantanamo Bay, Cuba, to bolster its case that Khalid Sheikh Mohammed and other al Qaeda detainees were complicit in the Sept 11, 2001 attacks.

The previously undisclosed material sheds light on the closed-door debate that shaped U.S. President Barack Obama’s response to what officials described as an agonizing foreign policy dilemma: how to allay Saudi concerns over a nuclear deal with Iran - Riyadh’s arch-rival - without exacerbating a conflict in Yemen that has killed thousands.

The documents, obtained by Reuters under the Freedom of Information Act, date from mid-May 2015 to February 2016, a period during which State Department officials reviewed and approved the sale of precision munitions to Saudi Arabia to replenish bombs dropped in Yemen. The documents were heavily redacted to withhold classified information and some details of meetings and discussion.

(A selection of the documents can be viewed here: tmsnrt.rs/2dL4h6L; tmsnrt.rs/2dLbl2S; tmsnrt.rs/2dLb7Ji; tmsnrt.rs/2dLbbIX)

An air strike on a wake in Yemen on Saturday that killed more than 140 people renewed focus on the heavy civilian toll of the conflict. The Saudi-led coalition denied responsibility, but the attack drew the strongest rebuke yet from Washington, which said it would review its support for the campaign to “better align with U.S. principles, values and interests.”

The State Department documents reveal new details of how the United States pressed the Saudis to limit civilian damage and provided detailed lists of sites to avoid bombing, even as officials worried about whether the Saudi military had the capacity to do so.

State Department lawyers “had their hair on fire” as reports of civilian casualties in Yemen multiplied in 2015, and prominent human rights groups charged that Washington could be complicit in war crimes, one U.S. official said. That official and the others requested anonymity.

During an October 2015 meeting with private human rights groups, a State Department specialist on protecting civilians in conflict acknowledged Saudi strikes were going awry.

“The strikes are not intentionally indiscriminate but rather result from a lack of Saudi experience with dropping munitions and firing missiles,” the specialist said, according to a Department account of the meeting.

“The lack of Saudi experience is compounded by the asymmetric situation on the ground where enemy militants are not wearing uniforms and are mixed with civilian populations,” he said. “Weak intelligence likely further compounds the problem.”

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The Saudis intervened in Yemen in March 2015 to restore President Abd-Rabbu Mansour Hadi after he was ousted by the Houthi rebels, whom Riyadh charges are backed by Iran. The Saudis gave Washington little advance notice, U.S. military leaders have said.

The Saudi government has called allegations of civilian casualties fabricated or exaggerated and has resisted calls for an independent investigation. The Saudi-led coalition has said it takes its responsibilities under international humanitarian law seriously, and is committed to the protection of civilians in Yemen. The Saudi embassy in Washington declined further comment.

In a statement issued to Reuters before Saturday’s attack, National Security Council spokesman Ned Price said, “U.S. security cooperation with Saudi Arabia is not a blank check. ... We have repeatedly expressed our deep concern about airstrikes that allegedly killed and injured civilians and also the heavy humanitarian toll paid by the Yemeni people.”

The United States continues to urge the Kingdom to take additional steps to avoid “future civilian harm,” he added.

NO-STRIKE LISTS

Since March 2015, Washington has authorized more than $22.2 billion in weapons sales to Riyadh, much of it yet to be delivered. That includes a $1.29 billion sale of precision munitions announced in November 2015 and specifically meant to replenish stocks used in Yemen.

In internal policy discussions, officials said, the Pentagon and the State Department’s Near East Affairs bureau leaned toward preserving good relations with Riyadh at a time when friction was increasing because of the nuclear deal with Iran.

On the other side, the State Department’s Office of the Legal Advisor, backed by government human rights specialists, expressed concern over U.S. complicity in possible Saudi violations of the laws of war, a former official said. Reuters could not determine the timing and form of that warning.

U.S. refueling and logistical support of Riyadh’s air force - even more than the arms sales - risked making the United States a party to the Yemen conflict under international law, three officials said.

About 3,800 civilians have died in Yemen, with Saudi-led airstrikes on markets, hospitals and schools accounting for 60 percent of the death toll, the United Nations human rights office said in August.

It stopped short of accusing either side of war crimes, saying that was for a national or international court to decide.

The White House convened a meeting in August 2015 on how best to engage the Saudis over rising civilian casualties, the emails show, in a sign of mounting concern over the issue. That same month, State Department officials gathered to discuss how to track those casualties.

In late January 2016, Deputy Secretary of State Antony Blinken chaired a meeting with officials across the department in part to discuss “Options to limit U.S. exposure to LOAC (Law of Armed Conflict) concerns,” according to a Blinken aide’s email.

The Law of Armed Conflict, a group of international laws and treaties, prohibits attacks on civilians and requires combatants to minimize civilian death and damage.

While preserving military ties with Riyadh, the Obama administration has tried to reduce civilian casualties by providing the Saudis with “no-strike lists” of targets to avoid, dispatching to Saudi Arabia a U.S. expert on mitigating civilian casualties and pressing for peace talks, the officials said.

“If we’re going to be supporting the coalition, then we have to accept a degree of responsibility for what’s happening in Yemen and exercise it appropriately,” a senior administration official said.

One no-strike list, called “The Overlay,” was delivered to the Saudis in mid to late 2015. It included water and electrical facilities and infrastructure vital to delivering humanitarian aid, a second senior official said.

“YOU CAN BE GUILTY”

In mid-October 2015, the White House ordered the U.S. Agency for International Development to compile a separate list of “critical infrastructure” that should be spared, a State Department email said.

Striking sites on the list could “do significant harm to Yemen’s ability to recover expeditiously” from the war, according to confidential U.S. talking points drafted the same month for use with Saudi officials.

“We urge you to exercise the utmost diligence in the targeting process and to take all precautions to minimize civilian casualties and damage to civilian infrastructure,” one talking point said.

After ceasefire talks collapsed in August and airstrikes resumed, coalition bombs destroyed the main bridge from the port of Hodeidah to the capital of Sanaa, a main supply route for humanitarian food aid, Oxfam International said.

Another U.S. official said the bridge was on a U.S. no-strike list. Reuters has not seen those lists.

In May, Washington suspended sales to Riyadh of cluster munitions, which release dozens of bomblets and are considered particularly dangerous to civilians, officials said.

More than 60 U.S. House of Representatives members are urging Obama to halt a new Saudi arms sale. An effort to block that sale failed in the U.S. Senate on Sept. 21.

Some critics say the administration’s approach has failed.

“In the law of war, you can be guilty for aiding and abetting war crimes and at some point the ... evidence is going to continue to mount and I think the administration is now in an untenable situation,” said Congressman Ted Lieu, a California Democrat and former military prosecutor.