By Fredrik Dahl
VIENNA (Reuters) - The United States said on Tuesday it is relying less on nuclear weapons as part of its defense planning, but Britain argued against banning such arms now.
Both powers, taking part for the first time in an international conference on the humanitarian impact of atomic bombs, spoke in support of pursuing the goal of a world free of such weapons of mass destruction.
The three other officially recognized nuclear weapon states -- Russia, France and China -- stayed away from the meeting. Many of the 157 states taking part are critical of what they see as slow progress on nuclear disarmament.
"We have reduced the role of nuclear weapons in the defense of our nation and our allies and it is the policy of the United States to not develop new nuclear warheads," U.S. non-proliferation envoy Adam Scheinman said.
He gave no more details on the reduced role.
However, in a possible allusion to tense ties with Russia, he added: "Progress requires a willing partner and a conducive strategic environment."
The United States and Russia hold more than 90 percent of the world's nuclear warheads.
The conference comes amid talk of a new Cold War between the West and Russia over the Ukraine crisis, during which President Vladimir Putin has pointedly stressed Moscow's nuclear arsenal.
Under the 1970 nuclear Non-Proliferation Treaty (NPT), the five recognized atomic bomb "haves" agreed to work towards eliminating their bombs, while the "have-nots" pledged not to pursue them. A treaty review conference is scheduled for 2015.
Pakistan and India, which both have nuclear weapons, have not signed the NPT. They attended the Vienna talks, as did Iran, which rejects Western accusations it too wants to build a bomb.
Israel, which is believed to have the Middle East's only nuclear arsenal and is also outside the NPT, did not attend.
Critics say there has been more emphasis on meeting the non-proliferation goal than getting the five major powers to fulfill their part of the deal. The five say that much progress has been made since the Cold War, with a sharp reduction in stocks.
Britain said the approach of those arguing for banning such bombs now or for fixing a timetable for their elimination failed "to take account of, and therefore jeopardizes, the stability and security which nuclear weapons can help to ensure".
"A declaratory ban, or a timetable not underpinned by the necessary trust, confidence and verification measures, would jeopardize strategic stability," Ambassador Susan le Jeune d'Allegeershecque added, advocating a step-by-step approach.
Mexico earlier called for a "diplomatic process leading to the negotiation and conclusion of a legally binding instrument for the prohibition of nuclear weapons".
(Editing by Angus MacSwan)