What are cluster bombs and why is US sending them to Ukraine?

The US has announced it is complying with a Ukrainian request to supply it with controversial cluster munitions.

The move has been criticised by human rights groups as the weapon is banned by more than 100 countries.

What are cluster munitions?

Cluster munitions are a method of dispersing large numbers of tiny bomblets from a rocket, missile or artillery shell that scatters them in mid-flight over a wide area.

They are intended to explode on impact but a significant proportion are "duds", meaning they don't explode initially - this happens especially if they land on wet or soft ground.