Bolton's remarks were a rare case of a U.S. official publicly speculating on the outcome of a bitter Security Council reform debate. In the past, he and other officials have repeated strong American opposition to rival proposals for adding at least 10 seats to the 15-nation body.
Bolton, who has made overhauling the United Nations a priority since President Bush appointed him to the job, said the world body must become more efficient, effective and accountable. Making the Security Council too large would undermine that goal, he said.
Increasing the number of seats to 25 or 26 "gives us great pause," he said, adding that the maximum that Washington could support would be 19 or 20 seats.
U.S. opposition is a key factor because there is no consensus among the 191 U.N. member states on how to expand the council. Also, while the United States does not have the power to block a vote in the General Assembly, where there are no vetoes, its support would be crucial when necessary changes to the U.N. Charter (search) would have to be approved by national legislatures.
Giving his first talk in Europe since taking his post in August, Bolton noted previous efforts to restructure the powerful Security Council had foundered.
"Our prediction would be that this latest effort at changing the composition of the council is not going to succeed," he said at the Chatham House foreign affairs think tank.
He reiterated the U.S. administration's support for Japan's bid to become a permanent member of the council, but did not say what other countries America might back.
While there is widespread support for expanding the council to reflect geopolitical changes since the U.N. founding 60 years ago, there is no agreement on how large it should be, who should get seats, whether new seats should be permanent or temporary, and who should have veto power.
The council currently consists of five permanent members with veto power — the United States, Britain, Russia, China and France — and 10 non-permanent members that serve two-year terms and have no power to block resolutions.
Japan teamed with Brazil, Germany and India in proposing a 25-member council, including six new permanent seats without veto power. All four hope to get a permanent seat and say the two others should go to Africa nations. The African Union, however, has insisted on veto power for its two seats and one additional non-permanent seat.
Another group, led by Italy and Pakistan, also calls for a 25-nation council but argues against making any of the new seats permanent and wants the 10 new members elected.
Neither group has been able to muster the required support from two-thirds of the 191 U.N. member states.
On other matters, Bolton reiterated that Washington is prepared to work outside the United Nations if the world body's flaws cannot be repaired.
"We look at it in a kind of cost-benefit way," he said. "If it's not solving problems, what do we do to fix it? And if we can't fix it, where else can we look to have those problems solved?"
Bolton's temper flared briefly when some in the audience laughed at his assertion that America's contribution to maintaining world security is a form of overseas aid.
"If you would prefer us to withdraw to Fortress America, get up and say so," he snapped.
"Yes," replied one man in the audience, but Bolton later said the United States would not retreat into isolationism.
In separate remarks, Bolton also said that Iran (search) has been trying to develop nuclear weapons for nearly two decades, and he urged the international community to take a firm stance on Iran's nuclear policy.
He told the British Broadcasting Corp. that Iran was "determined to get nuclear weapons deliverable on ballistic missiles that it can then use to intimidate not only its own region but possibly to supply to terrorists."
The envoy didn't elaborate on his claims but they were apparently based on U.N reports detailing more than 18 years of secret nuclear activities by Iran, including experiments that could be used in a weapons program.
However, Iran has repeatedly said its program is aimed toward peaceful uses of atomic power and not for weapons production.
In his radio remarks, Bolton said the difficult situation in Iraq should not weaken the United States' resolve on the Iran issue.
"There's no desire on our part to stay any longer (in Iraq) than is necessary," he said. "But it cannot be a situation where the difficulties that we encounter in Iraq stop us from taking the necessary steps against countries like Iran seeking nuclear weapons."