The US Government has stepped up a a campaign in Defence of the United Nations, which has become a key Republican target in Congress.
Susan Rice, US ambassador to the world body, condemned critics who have called for US funds to be held back while also attacking the United Nations for picking up "bad habits" including mismanagement and corruption.
Rice late Friday gave the first of a series of speeches around the United States, two days after administration supporters defeated a House of Representatives bill demanding repayment of surplus contributions to a UN fund.
Republicans have singled out US payments to the United Nations as part of their demands to get tough on cutting the massive US budget deficit.
"Main Street America needs the United Nations," Rice told the Oregon World Affairs Council in Portland, saying that it would be "immoral" and "dangerous" to be indifferent to new cross-border challenges.
"America can't police every conflict, end every crisis, and shelter every refugee. The UN provides a real return on our tax dollars by bringing 192 countries together to share the cost of providing stability, vital aid, and hope in the world's most broken places.
According to the ambassador, out of every dollar paid in tax by Americans 34 cents goes to social security and medicare, 22 cents to national security and just one tenth of a cent on UN dues.
Rice said the UN has "taken on huge responsibilities" for peacekeeping, saving refugees and preventing disease but also highlighted US concerns about mismanagement and isolated corruption.
"The truth is: the UN has also picked up some bad habits along the way, and we must continue to be clear about its shortcomings."
But she said it would be "short-sighted" to hold back funds from UN programs it does not approve of.
"Great and proud nations like ours are judged by their example," she said. "When we shirk our responsibilities, our influence wanes, and our standing is diminished."
"We just cannot lead from a position of strength when we're awash in unpaid bills."
The United States is the biggest contributor to the United Nations, paying more than 2.5 billion dollars to the UN peacekeeping and regular budgets in 2010.
President Barack Obama has vowed to pay all UN bills on time. However the United States owed about 736 million dollars in mandatory contributions at the end of 2010, a UN spokesman said in January