Belgium has pledged to donate 19m euro ($23m) to UNRWA, the UN's aid organisation for Palestinian refugees, after the US government announced it would slash its funding to the agency by half.

Deputy Prime Minister Alexander De Croo said in a statement on Wednesday that Brussels would allocate the funds over three years.

The first annual payment is being disbursed immediately "considering the financial difficulties that UNRWA currently faces", the statement said.

Washington announced on Tuesday it is withholding $65m out of the $125m aid package earmarked for the United Nations Relief and Works Agency for Palestinian Refugees, or UNRWA.

De Croo said the Belgian government stepped up in response to the urgent call for help by the commissioner-general of UNRWA, an agency that provides humanitarian help to millions of Palestinian refugees.

"I have a lot of respect for UNRWA's work, which has to operate in the most difficult and dangerous circumstances,” De Croo said in his statement.

"Living conditions in Gaza, Syria, the West Bank and elsewhere in the region are particularly tough,” he added.

"For a lot of Palestinian refugees the UNRWA is the last life buoy. With the help of UNRWA half a million of Palestine children are able to go to school. This prevents them from falling prey to radicalisation and extreme violence."

International condemnation

The US announcement on UNRWA's funding came after US President Donald Trump had threatened on January 2 to cut aid to Palestinians.

In a series of tweets, Trump had said: "... We pay the Palestinians HUNDRED OF MILLIONS OF DOLLARS a year and get no appreciation or respect.

"... With the Palestinians no longer willing to talk peace, why should we make any of these massive future payments to them?"

It's not only Pakistan that we pay billions of dollars to for nothing, but also many other countries, and others. As an example, we pay the Palestinians HUNDRED OF MILLIONS OF DOLLARS a year and get no appreciation or respect. They don’t even want to negotiate a long overdue... — Donald J. Trump (@realDonaldTrump) January 2, 2018

International NGOs sharply condemned the funding cut.

Kenneth Roth, executive director of Human Rights Watch, said in a Twitter post on Tuesday that Washington was "holding Palestinian kids' humanitarian needs hostage to political agendas".

Jan Egeland, secretary-general of the Norwegian Refugee Council, urged the US government to reverse its decision

Belgium, together with other EU member states, is now UNRWA's largest donor.

In the coming years, "Belgium, along with the EU and countries from the region, will continue to support UNRWA and the Palestinian refugees", De Croo's statement said.

Palestinian President Mahmoud Abbas is set to visit Brussels next Monday to exchange thoughts about Belgian's support.

UNRWA was established in 1949 in order to provide assistance to Palestinian refugees, who in the aftermath of the 1948 Arab-Israeli War numbered about 700,000.

The agency now offers support to approximately five million Palestinians in education, healthcare and housing, and is active in Palestinian occupied territories, Syria, Lebanon and Jordan.