The United Nations is facing flak from human rights groups for removing the Saudi Arabia-led military coalition from its report that accuses the coalition of being responsible for casualties that resulted from its bombing campaign in Yemen.

"This was one of the most painful and difficult decisions I have had to make." United Nations Secretary General Ban Ki-moon said on Thursday while addressing a press conference in New York. "It is unacceptable for member states to exert undue pressure," Ban added.

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Saudi Arabia is one of the UN's largest donors in the Middle East, giving hundreds of millions of dollars a year to UN food programmes in Syria and Iraq. In 2014, Saudi Arabia gave $500 million — the largest single humanitarian donation to the UN — to help Iraqis displaced by ISIS. Over the past three years, Saudi Arabia has also been become the third-largest donor to the UN's relief agency in Palestine, giving tens of millions of dollars to help rebuild Gaza and assist Palestinian refugees.

According to a report by Reuters, Riyadh had threatened to cut off funding if named by the UN. Reports also suggest the imposition of a Fatwa against the United Nations.

"Children already at risk in Palestine, South Sudan, Syria, Yemen and so many other places would fall further into despair," Ban Ki-moon told reporters.

The report, which was based on the work of UN researchers in Yemen, attributed 60 percent of the 785 children killed and 1,168 injured to the coalition's bombing. 

Meanwhile, according to another report by Reuters, Saudi Arabia has denied using threats. Saudi UN Ambassador Abdallah Al-Mouallimi described the UN figures as "wildly exaggerated."

"It is not in our style, it is not in our genes, it is not in our culture to use threats and intimidation. We have the greatest respect for the United Nations institution," Mouallimi told reporters shortly after Ban spoke.

Attempts at reconciliation are on and the report may be amended pending a joint review by the UN and Saudi Arabia.

UN actions slammed

According to Reuters, a letter to Ban on Wednesday, signed by groups including Human Rights Watch, Amnesty International and Oxfam, criticised the UN chief.

"If the Saudi-led coalition wants to be removed from the list, it should stop killing and maiming children and bombing schools and hospitals in Yemen — the violations for which it was listed," the groups wrote.

"The responsibility of the Saudi-led coalition for grave violations against children in many of these attacks is not in doubt," they added. "The evidence of grave violations against children in Yemen by the Saudi-led Coalition is overwhelming."

The Saudi-led coalition began a military campaign in Yemen in March last year with the aim of preventing Iran-allied Houthi rebels and forces loyal to Yemen's ex-president Ali Abdullah Saleh from taking power.

Some 6,000 people, about half of them civilians, have been killed in Yemen since last March, according to the UN.

The rights groups said Ban's decision to remove the coalition from the UN list undermined "an invaluable tool in efforts to curb violations against children in armed conflict."

"The list creates pressure on parties to armed conflict to comply with international law. Over 20 governments and armed groups have signed UN action plans and taken steps to end violations against children in order to be considered for 'de-listing'," they wrote.

Last year, the United Nations left Israel and the Palestinian militant group Hamas off the blacklist, after they had been included in an earlier draft, but criticised Israel over its 2014 military operations.

Analysts and political commentators believe the unintended consequences of such a decision may hurt the credibility of the United Nations in the long run.