Tensions between the U.S. and Russia escalated Monday as the Obama administration suspended talks over Syria’s civil war hours after Moscow announced it was ending cooperation with the U.S. on a 16-year-old program for the disposal of weapons-grade plutonium to curb the production of more nuclear bombs.

The Obama administration stopped pursuing diplomacy with Russia amid renewed attacks by Russian and Syrian forces on the city of Aleppo. Frustrated administration officials acknowledged that Syrian President Bashar Assad is making territorial gains with Moscow’s help after the collapse of a cease-fire negotiated by Secretary of State John F. Kerry and Russian Foreign Minister Sergei Lavrov.

“This is not a decision that was taken lightly,” State Department spokesman John Kirby said. “Unfortunately, Russia failed to live up to its own commitments and was also either unwilling or unable to ensure Syrian regime adherence to the arrangements to which Moscow agreed.”

At the White House, press secretary Josh Earnest said President Obama has run out of patience with Russian President Vladimir Putin.

“I think everybody’s patience with Russia has run out,” Mr. Earnest said. “There is nothing more for the United States and Russia to talk about with regard to trying to reach an agreement that would reduce the violence inside of Syria. And that’s tragic.”

The president’s spokesman acknowledged that Syrian and Russian forces have “enjoyed some limited gains” in the war by bombing civilian populations, while failing to advance Mr. Putin’s stated objective of crushing Islamic State extremists. Mr. Obama had previously predicted Russia would quickly get bogged down trying to help its ally in Syria’s brutal five-year civil war.

In another ominous move, Fox News reported Monday evening that Russia has deployed an anti-missile system in Syria for the first time, potentially as a means for the Assad government to counter U.S. and allied cruise missile attacks. Components of the SA-23 Gladiator anti-missile and anti-aircraft system, which has a range of roughly 150 miles, arrived over the weekend “on the docks” of a Russian naval base along Syria’s Mediterranean coastal city of Tartus, two U.S. officials told the network.

The U.S.-Russian talks were the main hope of the international community to ease the fighting in Syria, and United Nations special envoy Staffan de Mistura said Monday he “deeply regrets” the breakdown of the diplomatic push.

Mr. de Mistura said the U.N. will continue to push “energetically for a political solution” in Syria and “will never abandon the Syrian people to a destiny of endless violent conflict.”

As Mr. Obama’s thin hope of a diplomatic solution in Syria evaporated, the White House also said it was “disappointed” that Mr. Putin was pulling out of a joint agreement with the U.S. to reduce plutonium stockpiles.

Mr. Putin’s decree cited Washington’s “unfriendly actions” and the U.S. inability to fulfill its obligations under the 2000 deal as reasons for the move. Under the agreement, which was expanded in 2006 and 2010, Russia and the U.S. each were to dispose of 34 metric tons of weapons-grade plutonium, enough material for about 17,000 nuclear warheads.

Russia said it will keep the weapons-grade plutonium covered under the agreement away from weapons programs.

When it was signed, the deal was touted as an example of successful cooperation on nuclear nonproliferation between the onetime Cold War adversaries.

Russia said last year it had started up a plant that produces mixed-oxide commercial nuclear reactor fuel known as MOX from weapons-grade plutonium. Meanwhile, the construction of a similar U.S. plant in South Carolina has been years behind schedule and billions of dollars over budget.

The Obama administration wants to cancel the Savannah River Site’s MOX project and use an alternative method for disposing of excess plutonium.

Putin faults U.S.

Mr. Putin pointed to the stalled plant construction earlier this year when he accused the U.S. of failing to meet its end of the deal. He also argued that the policy change would give the U.S. “return potential,” or a chance to recycle the material back into the weapons-grade plutonium.

“Russia has been observing the agreement unilaterally for quite a long time, but now it no longer sees such a situation as possible amid the tensions,” Putin spokesman Dmitry Peskov said.

Sergei Kiriyenko, head of the state-controlled Rosatom nuclear corporation, said Monday that while MOX makes sure that weapons-grade plutonium can’t be used for any military purposes, the U.S. intention to dilute and stockpile the material means “it could be dug up again.”

Defending Mr. Putin’s move, the Russian Foreign Ministry said the U.S. has “done all it could to destroy the atmosphere encouraging cooperation,” citing U.S. sanctions on Moscow over the Ukrainian crisis and deploying NATO forces near Russian borders.

“We would like to bring Washington back to understanding that it can’t introduce sanctions against us in areas where it’s quite painless for the Americans, and at the same time continue selective cooperation in areas it sees as advantageous,” the Foreign Ministry said.

It emphasized that Moscow was suspending the deal and not annulling it altogether, adding it would be ready to restore the plutonium agreement if the U.S. takes Russian concerns into account.

In a draft bill on suspending the plutonium agreement sent to parliament, Mr. Putin specified the document could be restored if the U.S. reverses its moves to deploy its forces near Russia’s borders and pulls them back to areas in Europe where they were in 2000.

He added that the U.S. should also “renounce its unfriendly policies” by revoking anti-Russian sanctions and compensating Russia for the damage incurred, as well as by “putting forward a clear plan for the irreversible disposal of the weapons-grade plutonium in line with the agreement.”

Other U.S.-Russian nuclear deals still stand, including the pivotal New START nuclear arms reduction treaty that limited the number of deployed strategic nuclear warheads to 1,550 for each country.

The deteriorating relations between Moscow and Washington had more immediate implications for Syria, where the administration repeatedly has failed to secure Russia’s cooperation on fighting Islamic extremists, and the war, which includes both pro-Western Syrian rebel factions and radical Islamic jihadi groups such as Islamic State, has sparked a massive refugee crisis in Europe.

White House spokesman Mr. Earnest accused Mr. Putin of working mainly to prop up the Assad regime in the past year and of “supporting the Iranians in dropping bunker-busting bombs on civilian hospitals in Aleppo.”

He even suggested Mr. Putin, former head of the Russian KGB, was a coward for failing to attend the annual U.N. General Assembly gathering in New York City last month.

“President Putin didn’t even show up,” Mr. Earnest said, “in part because I think he rightly assumed that the rest of the international community was prepared to rebuke and condemn the actions that he has overseen” in Syria.

He said the U.S. would withdraw personnel that it had dispatched to take part in the creation of a joint U.S.-Russia center that was to have coordinated military cooperation and intelligence had the cease-fire taken hold. The suspension will not affect communications between the two countries aimed at conducting counterterrorism operations in Syria.

The State Department said Russia and the Syrian regime “have chosen to pursue a military course, inconsistent with the cessation of hostilities, as demonstrated by their intensified attacks against civilian areas, targeting of critical infrastructure such as hospitals, and preventing humanitarian aid from reaching civilians in need, including through the September 19 attack on a humanitarian aid convoy.”

An airstrike last month hit a United Nations humanitarian aid convoy, killing 20 people. The United States has accused Russia of hitting the convoy, but both Russia and Syria deny it.

Separately, The Associated Press reported that a close aide to al Qaeda’s top leader, Ayman al-Zawahri, had been killed in an airstrike while leading forces in Syria.

The Fatah al-Sham Front, formerly known as the Nusra Front, said via Twitter that Ahmed Salama Mabrouk, a veteran Egyptian jihadi known as Abu Farag al-Masri, was killed in a U.S.-led coalition airstrike in the northern Idlib province, according to the AP.

• This article is based in part on wire service reports.

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