After selling Saudi Arabia billion of dollars in weapons and intelligence to attack Yemen, the United States has reportedly warned Riyadh that it will reduce intelligence and military support for the kingdom unless necessary action is taken to limit civilian casualties.

US Defense Secretary James Mattis and General Joseph Votel, who oversees US military operations in the Middle East, are concerned that the US is helping Saudis and their allies, including the United Arab Emirates, with killing  civilians, unnamed officials familiar with the Pentagon’s view told CNN on Monday.

“At what point is enough enough?” one official said.

The Pentagon and the US State Department issued the warning following a Saudi-led airstrike on a school bus earlier this month, which killed 40 children and prompted widespread condemnations around the world.

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The UN says there is no justification for Saudi Arabia's latest massacre in Yemen which claimed the lives of scores of civilians, mostly of whom were children.

Democratic members of the US Congress have since written three separate letters to President Donald Trump, asking him to justify Washington’s continued support for Riyadh in the face of its many atrocities.

It was reported in the days following the attack that an American-made 500-pound (227 kilogram) laser-guided bomb was used to hit the bus.

The US has refused to publicly condemn the attack but, according to reports, Mattis has sent a top US general to talk to the Saudis about it.

“Lt. Gen. Garrett delivered a message of concern regarding the recent civilian casualty incident, and on behalf of the US government continued to urge for a thorough and expedited investigation as well as continued emphasis on the reduction of civilian casualties in the Yemeni campaign,” Rebecca Rebarich, a Pentagon spokeswoman told CNN.

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It is not yet clear whether Trump, who has been very supportive of the Saudis after inking a hefty $110 billion arms deals with the oil-rich kingdom last year, would allow a reduction of support for the Riyadh regime.

Former US President Barack Obama had banned the sale of American precision-guided military weapons to Saudi Arabia over “human rights concerns” but the ban only lasted until March 2017, when then-Secretary of State Rex Tillerson overturned it.

Besides the US, the UK and France have also been providing weapons and intelligence to Saudi Arabia and the UAE over the course of the unprovoked war, which has killed over 14,000 Yemenis, caused a deadly cholera outbreak and put the country on the verge of famine.