The United States placed a hold on the transfer of cluster bombs to Saudi Arabia over the latter’s use of this kind of weapon against civilians in its war on Yemen, a report said on Friday.

The United States placed a hold on the transfer of cluster bombs to Saudi Arabia over the latter's use of this kind of weapon against civilians in its war on Yemen, a report said on Friday.

Foreign Policy said the US did the move because it was "frustrated by a growing death toll" in Yemen war.

The news publication said on its website that the move is "the the first concrete step the United States has taken to demonstrate its unease with the Saudi bombing campaign in Yemen."

It quoted a senior US official, who cited reports that the Saudi-led coalition used cluster bombs “in areas in which civilians are alleged to have been present or in the vicinity.”

“We take such concerns seriously and are seeking additional information,” the official told FP on condition of anonymity.

The hold applies to CBU-105 cluster bombs manufactured by the US-based firm Textron Systems. According to Amnesty International and Human Rights Watch, Saudi-led forces have dropped CBU-105 munitions in multiple locations around Yemen, including Al-Amar, Sanhan, Amran, and the Al-Hayma port, FP said.

Cluster bombs contain bomblets that scatter widely and kill or injure indiscriminately. Sometimes bomblets fail to detonate immediately and can kill civilians months or even years later. The weapons were banned in a 2008 international treaty that arms sales giants, including the United States and Russia, refused to sign, according to Foreign Policy.

The report added that the US responded to humanitarian concerns by scaling back exports of cluster bombs and demanded changes in the munitions’ performance, such as banning those with a higher fraction of submunitions that do not explode on impact.

A 2009 U.S. law prohibits exporting cluster bombs that have a failure rate of above 1 percent. It also says the weapons cannot be used “where civilians are known to be present” and only against “clearly defined military targets.”

The move follows rising criticism by US lawmakers of America’s support for the oil-rich monarchy in the year-long conflict.

On Thursday, US senators introduced an amendment to the National Defense Authorization Act of 2016 aimed at placing additional conditions and Congressional oversight on future air-to-ground weapons sales to Saudi Arabia.

Washington has sold weapons and provided training, targeting information, and aerial refueling support to the Saudi-led coalition fighting in Yemen. It has also sold Riyadh millions of dollars’ worth of cluster bombs in recent years, FP reported.