What are cluster bombs, the US is expected to supply Ukraine and why are they controversial?

The Biden administration is set to announce a military aid package worth approximately $800 million (or €734 million) that will include cluster munitions for the first time, officials said.

The package is expected to be announced Friday and is likely to draw criticism from allies and humanitarian groups that oppose the use of cluster bombs.

Pentagon spokesman Pat Ryder said he didn't have any information when asked about the news but added that: "In the event we do provide this capability, we will be carefully selecting rounds with lower dud rates for which we have recent testing data."

A reduced "dud rate" means there will be far fewer unexploded rounds that can result in unintended civilian deaths.

Cluster bombs are weapons that open in the air, releasing submunitions, or "bomblets" that are dispersed over a large area.

They can be delivered by planes, artillery and missiles, according to the International Committe of the Red Cross.

Beyong the initial harm caused by the munitions upon impact, bomblets have a high rate of failure to explode, up to 40% in some recent conflicts, according to the ICRC.

That is why cluster bombs are controversial, with proponents of banning the bombs saying they kill indiscriminately and endanger civilians long after their use.