President Trump has vowed to block millions of dollars of funding for Palestinians, declaring on Twitter on Tuesday that they were “no longer willing to talk peace.”

Much of that aid flows through the United Nations Relief Works Agency for Palestinian Refugees, or Unrwa. While Mr. Trump did not explicitly mention the agency, his ambassador to the United Nations, Nikki R. Haley, made it clear that the agency was being looked at.

Asked specifically about Unrwa, Ms. Haley said: “He does not want to give any additional funding, or stop funding, until the Palestinians are agreeing to come back to the negotiation table.”

Many have expressed concern about Ms. Haley’s remarks. What is Unrwa, and how would a cut in American aid affect its work with more than five million Palestinian refugees?