John Bolton told the House International Relations Committee, the Associated Press reported, "It was an important first step in U.N. reform."
"It was not the alpha and the omega, but we never thought it would be the alpha and the omega."
The United Nations in mid-September approved a document that called for a Peacebuilding Commission to help countries emerging from conflict, but backed away from issuing other major reforms proposed by Secretary-General Kofi Annan in March.
Annan and Bolton, who was appointed Aug. 1 to the ambassadorship and lauded by President Bush as someone who could "provide clear American leadership for reform," have said they plan to continue pushing for more changes.
Some of the changes the international body may address at a later meeting include whether to expand the U.N. Security Council from 15 to 25 nations and to increase the permanent members with veto power over U.N. resolutions beyond the current five -- the United States, Britain, Russia, France and China, as Annan has proposed.
Weeks before the summit, which lasted from Sept. 14-16, dozens of member states argued over the wording in the document, which was intended to revive an organization that has been hobbled in recent years by scandals in its oil-for-food program and peacekeeping operations.
International Relations Committee Chairman Rep. Henry Hyde, R-Ill., who sponsored a bill aiming to enforce U.N. reforms by setting a timetable for specific changes and tying progress to payment of future U.N. dues, criticized the document.
The document's "lack of detail and definitive statements on critical areas such as oversight, accountability, management and budgeting do not inspire confidence," Hyde said, according to the AP.
Hyde's bill passed the House in June, but the Senate has yet to approve it. The White House opposes using the United States' $440 million in annual dues to pressure the United Nations, arguing it interferes with the president's ability to run foreign policy.
At the hearing, Bolton floated the idea of countries making voluntary payments to the United Nations. "The key is to break the sense of entitlement that permeates the U.N. system and allows these entities to expand and proliferate," he said in his written testimony.
But the U.N. chief of staff, Mark Malloch Brown, told Reuters that voluntary payments could "sink" the world body if countries were allowed to pick and choose how much they paid and which programs they supported.