President Vladimir Putin announced Monday that Russia would begin withdrawing the “main part” of its military from Syria, a surprise potential end to a six-month intervention that bolstered Syrian President Bashar al-Assad and dealt a grave blow to Syrian rebels.

The decision came as U.N.-

brokered peace talks between the Assad government and rebel representatives got underway in Geneva. The planned Tuesday start of the withdrawal coincides with the five-year anniversary of the beginning of street protests in Syria, an initially peaceful movement that was brutally repressed by Assad forces.

Through it all, Russia has backed Assad. But Monday’s decision may intensify pressure on the Syrian government to strike a deal with rebel groups in Geneva. Talks resumed there Monday after breaking down a month ago because the rebels were suffering such heavy losses in their surrounded stronghold of Aleppo. A shaky cease-fire has quelled fighting in Syria since late February, but Assad’s forces have continued an assault on their rivals.

“I hope that this will considerably increase the level of trust between all parties of the Syrian settlement and will contribute to a peaceful resolution of the Syrian issue,” Putin said in a meeting with his top deputies that was broadcast on Russian state television late Monday. In a separate phone call with Assad, Putin said the intervention had “radically changed the situation” on the ground, according to the Kremlin.

[How the Syrian revolt went so horribly, tragically wrong]

Putin said that Russia would keep open the Russian air force and naval bases in Syria but that the task of the Russian intervention had been achieved and diplomacy should take over.

The Obama administration was taken by surprise by the announcement, which the White House said President Obama later “discussed” with Putin in a telephone call that had been previously scheduled to talk about implementation of the cease-fire.

Putin made the decision unilaterally, without any such request from Assad, Kremlin spokesman Dmitry Peskov said. It was a pointed message suggesting that Russia’s support for Assad is not unlimited, now that he is unlikely to be deposed by force.

It was not immediately clear whether the announcement meant an end to all Russian airstrikes in Syria. The Kremlin spokesman said that Russia did not believe that issues with “terrorists” — the term Russia generally uses for all opponents of Assad — had been solved and that Russia intends to maintain a presence on the ground. Previous ­Russian announcements about peaceful intentions in Syria have been met with skepticism by Western nations.

After Assad appeared weakened and on the verge of defeat over the summer, the Russian intervention inverted the course of the conflict, paving the way with airstrikes for Assad’s ground forces. By February, shortly before the cease-fire went into effect, dozens of Russian bombers and jet fighters were often flying more than 60 sorties a day, according to Russia’s Defense Ministry, enabling major territorial gains by regime forces. Although Russian leaders said they were targeting the Islamic State and other “terrorists,” U.S. officials and rebels said the bulk of the airstrikes were being conducted against other rebel forces battling Assad, some of which were supported by the United States.

The mission was Russia’s first overseas combat deployment since the 1991 breakup of the Soviet Union, a major test for a military that in 2008 seemed stretched to the breaking point by a brief war in neighboring Georgia. Russia has sought to use the increased clout to play a bigger role at the negotiating table and to break through the international isolation that had settled on it after its 2014 annexation of Ukraine’s Crimean Peninsula.

Five years ago, few Syrians would have ever imagined that their uprising against their leader — a peaceful Arab Spring revolt — would turn into a violent proxy war for regional actors.

On March 14, Russian President Vladimir Putin announced that Russia would begin pulling its military from Syria, potentially winding down nearly six months of airstrikes. The alliance between Russia and the regime of Bashar al-Assad goes back decades. Here's a bit of historical context that explains why Russia was fighting to prop up its closest ally in the Middle East. (Ishaan Tharoor and Jason Aldag/The Washington Post)

On March 15, 2011, Syrians took to the streets in Damascus for unarmed rallies that would spread like wildfire across the country and would eventually be met with utter brutality by Assad’s security apparatus. Most Syrians back then would not have expected that the Islamic State and al-Qaeda-affiliated Jabhat al-Nusra would hijack their revolt and later provide the pretext for the Russian intervention.

For the first three months of the intervention, analysts and officials reported modest gains, as doubts about the battered Syrian army and militias loyal to Assad persisted. But in January, a Syrian offensive began scoring major victories, cutting off supply lines from Turkey and threatening Aleppo.

After helping broker the late February cease-fire, Russia pledged that it would push Assad forces to adhere to the deal.

[Obama thinks his Syria strategy is right — and folks just don’t get it]

The Obama administration had become increasingly frustrated in recent days over what it saw as Russia’s inability or unwillingness to press Syrian government forces to adhere to the cease-fire. In his call to Putin, the White House said, Obama welcomed the overall reduction in violence but “stressed that continuing offensive actions by Syrian regime forces risk undermining” both the truce and the political negotiations.

Late last week, the administration decided to publicly accuse Moscow of failing to rein in Assad, leading to a string of comments by officials including Secretary of State John F. Kerry, who on Sunday called on Putin to take control of Russia’s Syrian ally.

By signing on to the international agreement backing a cease-fire, he said, Russia and Iran “accepted responsibility for the forces that they control or influence. . . . So President Putin, who is invested in supporting Assad . . . should be somewhat concerned” by the actions of Syrian forces.

“We felt it was important, going into these talks, to make it clear that we weren’t blind to these violations, that they mattered, and that they really needed to stop,” a U.S. official said Monday.

Russian analysts said Putin’s announcement may be intended to press Assad at the talks after saving him on the ground.

As Assad representatives take a hard line in the talks, “I think that Russia is really not interested to fully take the responsibility for this behavior,” said Fyodor Lukyanov, a well-connected political analyst in Moscow who is the editor of Russia in Global Affairs.

It was unclear what effect the pullout would have on the negotiations. The U.N. envoy charged with the talks, Staffan de Mistura, told journalists ahead of Putin’s move that “the only Plan B available is the return to war, and to an even worse war than we had so far.”

Syrian opposition leaders on Monday offered cautious praise of the pullout decision.

“For us, it’s important to see actions instead of hearing words,” said Salem al-Muslet, a spokesman for the main opposition group, the High Negotiations Committee. “If this decision actually removes all Russian troops from Syria, then this will be a positive step.”

Naylor reported from Geneva. Karen DeYoung in Washington and Andrew Roth in Moscow contributed to this report.

Read more:

Syria truce wobbles amid claims of government attacks, aid-delivery snags

Nanny accused of beheading Moscow child cites revenge for Syria airstrikes

As energy prices drop, ordinary Russians are protesting

Today’s coverage from Post correspondents around the world