Leaders of the Global Zero movement applauded the new START treaty signed Thursday by presidents of U.S. and Russia, while urging the two nuclear super powers to further reduce their nuclear arsenals.

Global Zero, an international initiative launched in December 2008 to promote the elimination of nuclear weapons, held a press conference on the new START treaty and the upcoming Nuclear Security Summit.

"I was delighted to see the presidents not only focus on important steps that are contained in this treaty, but also focus on the need of follow-on negotiations to further reduce those weapons and continue toward the long-term goal of nuclear elimination," said Richard Burt, U.S. chief negotiator of START 1 and Global Zero U.S. Chair.

U.S. President Barack Obama and his Russian counterpart Dmitry Medvedev signed a successor treaty to the expired Strategic Arms Reduction Treaty (START) on Thursday, agreeing to slash both nations' arsenals of strategic nuclear warheads to 1,550 over seven years, about a third less than currently permitted.

While recognizing the significance of the new treaty, leaders of Global Zero believed that it only represented a small step compared with the long trek toward their ultimate goal -- a world free of nuclear weapons.

Burt called on "the two leading nuclear powers to demonstrate their support and to be credible to go down to low enough levels so that other countries are enticed into the process."

In February, Global Zero rolled out an action plan on how to get to zero nuclear weapons, under which the U.S. and Russia should reduce their nuclear warheads to 1,000 each before other nuclear weapons states can be involved in the disarmament process.

The new START treaty kicked off a busy nuclear season. Nuclear Security Summit, which is mainly to address the issue of safeguarding nuclear materials and fighting nuclear terrorism, is scheduled to be held on April 12-13 in Washington, and this will be followed by Nuclear Non-proliferation Treaty review conference in May.

Leaders of Global Zero called for strong commitments by the 47 leaders participating in the Nuclear Security Summit.

"Terrorist groups have been trying to buy, build or steal arms. In the last two decades, there have been at least 25 incidents of nuclear explosive materials being lost or stolen," said Valerie Plame, former CIA covert officer specializing in nuclear counter proliferation.

"There is enough highly enriched uranium -- HEU -- in the world today to build more than 100,000 bombs. Terrorists looking to buy or steal HEU could look to the approximately 40 countries with nuclear weapons materials," she added.

Plame believe that "the only way to eliminate the threat of nuclear weapons terrorism is to lock down all nuclear materials and eliminate all nuclear weapons in all countries, global zero."