NATO leaders are unlikely to agree to reduce the defense organization's tactical nuclear stockpiles when they meet later this month, defense officials said Saturday.

The leaders are to meet in Lisbon on November 19-20 to map out the future of the 61-year-old alliance. Russian President Dmitry Medvedev will also attend the talks expected to touch on missile defense.

The nuclear arsenal across Europe remains a source of friction with Russia and within NATO -- with Germany, Poland and Sweden calling for a greater NATO commitment to nuclear disarmament resisted by France and the United States.

But a revised mission statement is not likely to address US nuclear weapons in Europe originally meant to deter a Russian invasion, according to defense officials.

"As long as the world is nuclear, the (NATO) alliance has to keep nuclear weapons," Stephane Abrial, Supreme Allied Commander for NATO transformation told a security conference in Halifax, Canada.

Canadian Defense Minister Peter MacKay earlier told the daily Globe and Mail: "I think it's unlikely you're going to see the non-proliferation movement gain momentum unless there's a willingness on both sides."

US Undersecretary of Defense Michele Flournoy, meanwhile, noted that a new Strategic Arms Reduction Treaty (START) negotiated earlier this year restricts Russia and the United States to a maximum of 1,550 deployed warheads each, a cut of about 30 percent from a limit set in 2002.

"Nonstrategic nuclear weapons need to be a focus of future negotiations," she said, but added: "That's a long-term prospect. Historically, those weapons have been off the table in negotiations. We want to put them on the table as a key topic of discussion with Russia beyond the new START treaty."

"We need to start with transparency, more full accounting of what the Russians have in the vicinity of Europe, for example. We need to try to get to an agreement on pulling those weapons back from the NATO border and the Russian border."

In April, US Secretary of State Hillary Clinton appeared to rule out early withdrawal of an estimated 240 US nuclear weapons stored in NATO nations Belgium, Germany, Italy, the Netherlands and Turkey -- weapons that would be carried by bomber aircraft.