WASHINGTON—The U.S. and Russia said they have fulfilled obligations under a 2010 treaty to reduce the number of strategic nuclear warheads, meeting the requirement by a Monday deadline.
The two governments made the declaration despite worsening relations, but still face the task of renewing the pact when it expires in three years amid a dispute over another weapons agreement.
As ties have frayed over conflicts in Syria and Ukraine and amid allegations of Russian interference in the 2016 presidential election, U.S. officials said Monday’s announcements were a positive sign.
The State Department said the U.S. has fulfilled its commitments under the New Strategic Arms Reduction Treaty, or New START, which was signed by former President Barack Obama in 2010 and ratified by the Senate in 2011.
In Moscow, the Kremlin said it, too, had complied with the reductions to which it agreed under the accord. “The Russian Federation has fully complied with its commitment to reduce its strategic offensive weapons.”
The treaty limits both Washington and Moscow to no more than 1,550 strategic nuclear warheads, and both must observe limits on other forms of weapons and delivery systems, such as bombers and submarines. Monday was the deadline for meeting the limits.
State Department spokeswoman Heather Nauert said the treaty’s implementation makes strategic relations “more stable, transparent, and predictable; critically important at a time when trust in the relationship has deteriorated, and the threat of miscalculation and misperception has risen.”
Both countries also allow each other to conduct on-site inspections to ensure treaty obligations are met. The State Department said in a fact sheet it has conducted 252 on-site inspections since the treaty took force in 2011.
U.S. officials added they expect to find Russia in compliance when the two countries exchange data on their strategic nuclear arsenals within the next month. Once officials confirm both sides have met the limits, the treaty is considered implemented.
While Russia is complying with the New START treaty, the U.S. alleges Moscow is violating a separate 1987 accord banning American and Russian intermediate-range missiles based on land. The alleged violation concerns the deployment of a Russian ground-launched cruise missile. If the dispute over this violation isn’t resolved, it may color the American debate on whether to extend New START when it expires in 2021.
In announcing that it was adhering to the New START treaty, the Russian foreign ministry complained about some of the technical procedures the U.S. has employed to meet the accord’s limits.
The U.S., for example, has taken missiles out of four of the 24 tubes on its Ohio-class submarines and modified the tubes so they can’t launch missiles, arms-control experts say. The Russians have complained that this conversion could be reversed. The State Department declined to discuss the issue on Monday, but U.S. officials have maintained that the procedures used are adequate and cost-effective.
“Russia insistently urges the U.S. to continue the constructive search for mutually acceptable solutions,” the Russian foreign ministry said.
The New START pact expires in 2021, leaving its renewal in question as mistrust simmers between the two countries. A Kremlin spokesman said on Monday that the U.S. and Russia are in contact through diplomatic channels on potentially extending the treaty, which can be prolonged for as long as five years by mutual consent.
“New START implementation is a significant accomplishment,” said Daryl G. Kimball, executive director of the Arms Control Association, which advocated for the treaty a decade ago. He said the treaty succeeded in trimming “still oversized nuclear arsenals.”
“The next step is for Presidents [Donald] Trump and [Vladimir] Putin to agree to extend the treaty for another five years—to 2026—to avert unconstrained strategic nuclear arms race between the world’s two largest nuclear actors,” Mr. Kimball said.
An extension wouldn’t require Senate ratification, according to the treaty.
Mr. Trump ordered a review of nuclear programs and strategy shortly after taking office. A new plan publicly outlined last week calls for a upgrading the U.S. arsenal and includes development of two new sea-based nuclear weapons to respond to Russia and China’s growing military capabilities.
—James Marson in Moscow
contributed to this article.
Write to Felicia Schwartz at Felicia.Schwartz@wsj.com