The US state department has approved the sale of $1.3bn of bombs to Saudi Arabia, to allow the Gulf nation to replenish its depleted weapons supplies as it continues to conduct intensive air strikes in Yemen and Syria.
Amnesty International, however, told Al Jazeera on Tuesday that some of the types of bombs the US proposes to sell have previously been used in air strikes in Yemen that violated international humanitarian law.
The Defense Security Cooperation Agency (DSCA) released a statement on Monday, saying that the state department approved the sale of about 22,000 smart and general purpose bombs to Saudi Arabia.
Congress was notified of the proposed sale last week, triggering a 30-day period where the legislators have the ability to block the sale.
"The purchase replenishes the Royal Saudi Air Force's current weapons supplies, which are becoming depleted due to the high operational tempo in multiple counter-terrorism operations," a DSCA statement said.
"This acquisition will help sustain strong military-to-military relations between the United States and Saudi Arabia, improve operational interoperability with the United States, and enable Saudi Arabia to meet regional threats and safeguard the world's largest oil reserves."
The US is a major supplier of military equipment to Saudi Arabia, with more than $90bn of sales recorded between 2010 and 2015.
Saudi Arabia has been leading a coalition air campaign against Yemen's Houthi rebels since March, seeking to reinstate exiled President Abd-Rabbu Mansour Hadi and his government.
While leading the coalition, Saudi Arabia has also been a large donor to aid efforts in the stricken country, donating hundreds of millions of dollars to fund humanitarian programmes - including $274m to the UN's Yemen Flash Appeal.
Amnesty International released a report last month calling on states like the US to stop selling certain arms to Saudi Arabia, after it investigated a number of air strikes in Yemen, where civilians were killed, and found evidence of what it called unlawful air strikes which could amount to war crimes.
Amnesty's Yemen researcher Rasha Mohamed told Al Jazeera that of the bombs that the US proposes selling to Saudi Arabia, the group opposes the sale of three types because they have previously been used in unlawful strikes in Yemen.
The three types listed by Mohamed are the BLU-117/MK-84 2000lb general purpose bombs, BLU-110/MK-83 1000lb general purpose bombs and BLU-111/MK-82 500lb general purpose bombs.
Combined, the US is proposing to sell more than 10,000 of the three bomb types to Riyadh.
Mohamed added that another type of bomb on the DSCA list - GBU-24 Paveway III laser guided bombs - had been linked to an air strike in a residential area in Yemen which Amnesty believes "involved international humanitarian law violations".
"Since the conflict in Yemen began eight months ago, more than 2,000 civilians - including at least 400 children - have been killed, and the vast majority of civilian deaths and injuries have been caused by the Saudi Arabia-led coalition's often reckless and indiscriminate aerial bombing campaign," Mohamed said.
"Many of the strikes that Amnesty International has researched were unlawful - in that they deliberately targeted civilian objects or disproportionately harmed civilians and civilian objects in relation to the expected military gain from the strike, or failed to distinguish between these and military objectives."
Mohamed said that under the Arms Trade Treaty, which came into force in late 2014 and to which the United States is a signatory, a state is not allowed to sell weapons to a party to an armed conflict, if it knows the arms could be used "in attacks directed against civilians and civilian objects, or any other war crime as defined by international agreements to which the state is a party".
"There have been no investigations to date and no accountability for unlawful strikes in Yemen. It is therefore unclear under what basis the US is continuing to supply Saudi Arabia with arms and munitions which have previously been used in unlawful strikes," she said.
Another rights watchdog, Human Rights Watch, in August accused Saudi forces of using unlawful cluster bombs in Yemen.
The claim was denied by a spokesman for the coalition.
Amnesty International said earlier this year that both sides fighting in the war have routinely used weapons such as Grad-type rockets, mortars and artillery shells in densely populated residential areas.
The Houthis have also been accused of other atrocities, such as using civilians as human shields and recruiting child soldiers.
Yemen's conflict pits the Houthis and troops loyal to former president Ali Abdullah Saleh against forces including southern separatists, tribal fighters and troops loyal to President Hadi.
Along with its ongoing campaign in Yemen, Saudi Arabia has also been taking part in air strikes against the Islamic State of Iraq and the Levant (ISIL) in Syria.
With reporting by Mark Worley in Doha