Focusing on the deteriorating humanitarian situation in the Yemen- some nine million people are believed to be in need of assistance and hundreds of thousands have been displaced - the letter says that a pause in the bombing by the Saudi-led coalition will allow much needed emergency supplies to enter the country, IHRC reports.
Targeting of the country's airports and runways and a military blockade of Yemen by coalition forces have made impossible the delivery of the required amounts of aid.
"Human Rights Watch new evidence shows Saudi-led coalition is using internationally banned cluster bombs in Yemen."
The letter also draws the UN secretary general's attention to the continuing presence of Saudi Arabia on the UN Human Rights Council, a position which the signatories say us inconsistent with the numerous violations of human rights and international law Riyadh is committing in respect of Yemen.
The coalition has declared whole cities and towns to be military targets, something the signatories say amounts to collective punishment which is a war crime, as it is disproportionate in scale and fails to distinguish between military targets and the sanctity of civilians and civilian infrastructure.
Human Rights Watch on Sunday published new evidence alleging a Saudi-led coalition is using internationally banned cluster bombs in Yemen, urging it to stop such attacks that were harming civilians.
The New York-based watchdog said it documented the use of three types of cluster munitions in Yemen, where Saudi-led warplanes have pounded Yemen since March 26.