State Department spokeswoman Heather Nauert says the US cannot be expected to be "the chief donor to every organisation around the world".
The United States will not provide $45 million in food aid for Palestinians that it pledged last month as part of the West Bank/Gaza Emergency Appeal led by the UN Relief and Works Agency (UNRWA), the State Department said on Thursday.
The State Department had said on Tuesday that Washington would withhold a separate $65 million it had planned to pay the UN agency that serves the Palestinians, saying UNRWA needed to make unspecified reforms.
State Department spokeswoman Heather Nauert denied the withholding of the $65 million was to punish Palestinians, who have been sharply critical of Trump's announcement last month that he would move the US Embassy to Jerusalem from Tel Aviv.
In a December 15 letter to UNRWA Commissioner-General Pierre Krahenbuhl, State Department Comptroller Eric Hembree had pledged $45 million to the West Bank/Gaza Emergency Appeal.
"The United States plans to make this funding available to UNRWA in early 2018," according to the letter, seen by Reuters on Thursday.
"An additional letter and contribution package confirming this contribution will be sent by or before early January 2018."
The United States had made clear to UNRWA that the $45 million was a pledge aimed at helping the agency with "forecasting," but it was not a guarantee, Nauert told reporters at a regular State Department briefing.
"At this time, we will not be providing that, but that does not mean – I want to make it clear – that does not mean that it will not be provided in the future," Nauert said.
She repeated the US view that UNRWA needs reform, saying there are a lot more refugees in the programme than previously, and that "money coming in from other countries needs to increase as well to continue paying for all those refugees,"
"So we're asking countries to do more," Nauert said.
"Fundamentally, we just don't believe that we have to be the chief donor to every organisation around the world."
Despite the decision on the food aid pledge, she said: "We are the most generous country on the planet. We continue to be."
Trump said in a Twitter post on January 2 that the United States gives the Palestinians hundreds of millions of dollars a year, "but get no appreciation or respect."
The decisions to curb funding are likely to compound the difficulty of reviving Israeli-Palestinian peace talks as well as further undermine faith that the United States can act as an impartial arbitrator.
The last talks collapsed in 2014, partly due to Israel's opposition to an attempted unity pact between the Palestinian factions Fatah and Hamas, and because of Israeli settlement building on occupied land that Palestinians seek for a state, among other factors.