A U.S.-Russia nuclear arms treaty that limits the number of atomic warheads the former Cold War foes can possess and allows them to inspect each other's arsenals _ securing a key foreign policy goal of President Barack Obama _ went into effect Saturday.

The New START treaty was approved by the U.S. Senate in December after Obama pressed strongly for its passage. Russia ratified the deal last month.

The 10-year treaty, which can be extended by another five years, went into effect when U.S. Secretary of State Hillary Rodham Clinton exchanged the ratification papers with Russian Foreign Minister Sergey Lavrov on the sidelines of an international security conference in Munich.

New START is a cornerstone of Obama's efforts to "reset" U.S. relations with Russia, and Clinton called it a "milestone in our strategic partnership."

"When it comes to the button that has worried us the most over the years _ the one that would unleash nuclear destruction _ today we take another step to ensure it will never be pushed," Clinton told reporters after the treaty went into effect.

Lavrov said that the treaty is in the national interests of both Russia and the United States.

"Both Russia and the U.S. share responsibility for security in the whole world," he said through a translator.

New START, negotiated last year, limits each side to 1,550 strategic warheads, down from 2,200. It limits the number of deployed strategic launchers and heavy bombers to 700.

The pact also re-establishes a monitoring system that ended in December 2009 with the expiration of an earlier arms deal. Russia and the U.S. have the right to conduct onsite inspections beginning 60 days from the agreement going into effect Saturday.

U.N. Secretary-General Ban Ki-moon hailed the treaty's entering into force as "a historical, political milestone on the road to our ultimate goal: achieving a world free of nuclear weapons."

Looking ahead, Clinton said the U.S. is in talks with Russia about how the two countries can further work together to address issues that affect their common security, while maintaining strategic stability.

Suggestions include joint analysis, joint exercises, and sharing of early warning data that could form the basis for a cooperative missile defense system, Clinton said.

She said she would also talk with Lavrov about "further arms control issues, including non-strategic and non-deployed nuclear weapons and our ongoing work to revive, strengthen and modernize the regime on conventional forces."

Lavrov called New START "a product of the understanding that unilateral approaches to security are counterproductive."

"The principles of equality, parity, equal and indivisible security ... form a solid basis for today's Russian-American interaction in a range of areas," Lavrov said.

"The treaty that enters into force today will enhance international stability."

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Geir Moulson contributed to this story.