The Saudi-led coalition air campaign is using cluster bombs against Houthi rebels in Yemen, according to Human Rights Watch.

The controversial munitions have been banned by more than 100 countries and could pose a dangerous threat to civilians in a conflict that has already killed at least 1,200 in recent weeks, according to the U.N.

HRW provided video and photographic evidence that the Saudis used cluster munitions.

The bombs were reportedly found in Yemen's northern region of Saada, a stronghold for the Houthi rebels and a region that borders Saudi Arabia. Video of the area show the bombs falling with distinctive white parachutes. According to HRW the cluster bombs fell in an area near four to six "clusters" of villages.

The controversial bombs pose dangers to civilians and 116 countries agreed to not use the deadly devices in a 2008 treaty. However the US, Saudi Arabia and Yemen did not sign the treaty. Cluster ammunition scatters submunitions or "bomblets" over a wide area and can pose a threat to civilians even after a conflict is over. Up to 30 percent of the ammunition don't explode immediately meaning they can be a danger to civilians for years, including curious children who pick up the sometimes colorful cylinders thinking they are toys.

Human Rights Watch describes the bombs as "de facto landmines."

According to the Associated Press, Saudi Arabia has previously denied using cluster bombs in their weeks-long fight against Houthi rebels.

The U.S. State Department did not have an immediate statement on the reported use of cluster bombs.

The news comes the same day that Yemeni officials reported that a small ground force of Saudi-led troops arrived in the southern city of Aden, according to the AP.

At least 20 Saudi troops were reportedly in the southern port city of Aden on a reconnaissance mission and no fighting was reported between them and Houthi rebels, according to the AP. Aden has seen intense fighting since Houthis arrived in the city weeks ago to battle with troops still loyal to Yemeni President President Abed Rabbo Mansour Hadi.

Coalition leaders had said previously that a ground offensive would follow the weeks of airstrikes. The Saudi-led coalition started airstrikes after Houthi rebels swept through much of the country, including the city of Aden where Hadi had taken refuge after Houthis took control of the Yemeni capital of Sanaa last fall. Hadi escaped to Saudi Arabia in March.

Last week United Nations Secretary-General Ban Ki Moon released a statement urging all parties to protect civilians and civilian infrastructure in the city.

"There are credible reports of families in Aden trapped by the bombardment and snipers targeting civilians in the street," the statement said. "Attacks on civilians and civilian infrastructure, including hospitals, humanitarian warehouses and UN compounds, are unacceptable and in violation of international humanitarian law."