Russian President Vladimir Putin ordered a partial pullout of the Russian military from Syria on Monday, after saying his troops largely achieved their combat goals in the country.

He also ordered the country’s diplomatic efforts be stepped up to secure a peace deal in Syria. The move was announced on the day U.N.-backed peace talks on Syria resumed in Geneva.

Announcing his decision in a televised meeting with Russia's foreign and defense ministries in the Kremlin, Putin said the Russian air campaign has allowed Syrian President Bashar Assad's military to turn the tide of war and helped create conditions for peace talks.

"With the tasks set before the Defense Ministry and the military largely fulfilled, I'm ordering the Defense Minister to start the pullout of the main part of our group of forces in Syria, beginning tomorrow," he said.

Putin didn't specify how many planes and troops should be withdrawn. He emphasized that the Russian airbase in Hemeimeem in Syria's coastal province of Latakia and a naval facility in the Syrian port of Tartous will continue to operate.

The number of Russian soldiers in Syria has not been revealed.

The U.N. special envoy for Syria, Staffan de Mistura, who restarted peace talks between the Syrian government and the opposition in Geneva on Monday, said he had no comment on Putin's announcement when contacted by The Associated Press.

Earlier in the day, he warned that the only alternative to the negotiations is a return to war, and described political transition in the country as "the mother of all issues."

As peace talks opened in Geneva, President Obama spoke with President Vladimir Putin about the withdrawal and discussed "next steps" in upholding the fragile truce that has quieted the country's civil war, the White House said.

In a statement about the phone call, the White House said Obama told Putin he welcomed the "much-needed reduction in violence" since the cease-fire took effect late last month. But the president also noted the Syrian regime's continued "offensive actions" threaten to break the deal and could undermine plans for a United Nations-led political process.

"The President underscored that a political transition is required to end the violence in Syria," the White House said.

The two leaders spoke shortly after Putin announced the partial withdrawal. For nearly six months, Russia has mounted an air campaign backing up the Syria President Bashar Assad's battle against opposition forces. The White House statement did not offer the president's reaction to Putin's move and instead focused on what else Russians could do to use its influence with the Assad regime.

The White House said Obama noted "some progress" on getting humanitarian aid to Syrians, but added that regime forces continue to impede access to some areas, particularly Daraya.

U.S. officials told Reuters they had no advanced warning of Putin's decision to withdraw the main part of Russian forces from Syria, nor had they seen any indications of preparations for such a withdrawal.

One rebel group told Reuters they did not understand the Russian announcement.

"It's a surprise, like the way they entered the war. God protect us," Fadi Ahmad, spokesman for the First Coastal Division, a Free Syria Army group fighting in the country's northwest, told Reuters.

The Syrian presidency said Assad and Putin spoke on the phone Monday and jointly agreed that Russia would scale back its forces in Syria. It rejected speculation that the decision reflected a rift between the allies and said the decision reflected the "successes" the two armies have achieved in fighting terrorism in Syria and restoring peace to key areas of the country.

The Syrian army said it would continue its operations against the Islamic State group, the Nusra Front and other terrorist organizations "with the same tempo."

The Russian and U.S.-brokered cease-fire that began on Feb. 27 has largely held, but both the Syrian government and its foes have accused one another of violations. The Islamic State group and Al Qaeda's branch in Syria, the Nusra Front, are excluded from the cease-fire.

Putin said Monday's move would send a "good signal" to the parties to the conflict, help raise trust and help serve as a stimulus for Syria's political talks. The Kremlin said the president coordinated the move with Assad.

Putin added that the Russian troops will continue to oversee the observance of the Russian- and U.S.-brokered cease-fire.

Moments before meeting with a Syrian government envoy in Geneva, de Mistura laid out both high stakes and low expectations for what is shaping up as the most promising initiative in years to end the conflict that moves into its sixth year on Tuesday.

At least a quarter of a million people have been killed and half of Syria's population has been displaced, flooding Europe with refugees.

The Geneva talks come as the truce helped vastly reduce the bloodshed and allowed the recent resumption of humanitarian aid deliveries to thousands of Syrians in "besieged areas" — zones surrounded by fighters and generally cut off from the outside world.

De Mistura laid out a stark choice for Syrian parties in the talks, saying: "As far as I know, the only plan B available is return to war — and to even worse war than we had so far."

The two sides are deeply split on Assad's future. His foreign minister, Walid al-Moallem, said Saturday that any talk of removing Assad during a transitional period sought by the U.N. is "a red line," and rejected the international call for a presidential election to be held within 18 months — a key demand of the opposition.

But de Mistura, keeping to language laid out in the U.N. Security Council resolution in December that paved the way for the talks, insisted that political change, including a timetable for new elections within 18 months, is the ultimate goal.

"What is the real issue — the mother of all issues? Political transition," he said.

Assad, however, has announced that parliamentary elections in Syria will go ahead next month according to schedule. A Syrian official, Hisham al-Shaar, said the elections will be held only in areas under government control and there will be no polling stations in Syrian embassies abroad or in refugee camps.

On Monday, as the election campaign officially kicked off, streets in the capital Damascus were festooned with electoral banners and posters of hundreds of government-approved candidates.

In the so-called proximity talks, the two sides don't meet face to face, but meet separately with de Mistura and his team, who shuttle back and forth.

The talks began Monday with de Mistura hosting a government delegation led by Syria's U.N. ambassador, Bashar Ja'afari. Speaking to reporters afterward, Ja'afari called the meeting "positive and constructive" and said the government delegation "submitted ideas and views" for a political solution to the crisis. He said the opposition will meet de Mistura on Tuesday, and his delegation would meet again on Wednesday.

The talks have shaped up as the best, if distant, chance in years to end a war that has created an opening for radical groups including Islamic State and the Al Qaeda-backed Nusra Front to gain large swaths of land, and prompted at least 11 million people to leave their homes — many fleeing abroad to places like Turkey, Jordan, Lebanon and Iraq, as well as to Europe.

The Associated Press contributed to this report.

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