A Turkish policeman waits at a security check near the Summit zone before the G20 Summit in Antalya,Turkey. (Tolga Bozoglu/European Pressphoto Agency)

President Obama vowed Friday “to go after any terrorist networks that go after our people” after gruesome attacks in Paris jolted the White House just as Obama was preparing to rally world leaders behind his latest plans to combat radical Islamist fighters.

In a hastily arranged appearance before reporters, Obama called the attacks that killed more than 140 people an “outrageous attempt to terrorize innocent civilians” and said the murders represented “an attack on all of humanity and the universal values that we share.”

It was not immediately clear whether the events in Paris would alter Obama’s plans to depart Washington on Saturday afternoon for a nine-day trip to Turkey, the Philippines and Malaysia for global security and economic summits.

But the violence on the streets of Paris added new urgency ahead of the two-day Group of 20 summit in Antalya, Turkey, where Obama was hoping to build support for his administration’s strategy to defeat the Islamic State in the Middle East and bring a negotiated end to the civil war in Syria.

Coincidentally, Obama had spoken with French President François Hollande by phone before the attacks Friday to discuss the summit’s agenda. Obama called France an “extraordinary counterterrorism partner” and pledged that the United States would offer similar succor in the wake of the Paris killings.

“We want to be very clear that we stand together with them in the fight against terrorism and extremism,” Obama said.

The events of the past two weeks, including the downing of a Russian airliner in the Sinai Peninsula that killed all 224 people on board, have highlighted the immense challenge facing international leaders who have struggled to coalesce around a coordinated strategy to respond to terrorist attacks in the Middle East and elsewhere.

Obama’s aims to consolidate support under American leadership have been tested by a newly empowered Vladimir Putin of Russia, who is also scheduled to attend the summit. Although Obama has scheduled no formal meetings with Putin, both the White House and the Kremlin said they expected them to meet on the sidelines.

The Obama administration in recent weeks has escalated its efforts on the diplomatic and military fronts in Syria, including an initiative to enlist Russia and Iran — both backers of Syrian President Bashar al-Assad — in a negotiated political transition that would replace Assad with a transitional government formed of regime remnants and opponents who have been battling for more than four years to oust him.

Putin, whose warplanes began bombing the Syrian opposition last month just as Iran sent forces to bolster Assad’s military, has his own ideas about how to settle the conflict. Putin has cast himself in competition with Obama for a leadership role in resolving Syria’s problems.

In an interview Friday with Russian and Turkish media, Putin said other nations have “no right” to demand that Assad leave office or to fashion a political solution for Syria from the outside.

“Only those who believe in their exceptionality allow themselves to act in such a shameless manner and impose their will on others,” he said, according to the Russian news agency Interfax, in a clear reference to the United States.

The interplay between the two, how other leaders respond to them, and whether there is any agreement on a way forward in Syria will be a focus of summit attention. The Obama administration has not signaled a potential major breakthrough.

The last meeting between Obama and Putin, during the U.N. General Assembly meetings in New York two months ago, produced an awkward handshake and little cooperation. A day later, Putin stunned the administration by sending troops into Syria and launching airstrikes, mostly aimed at the rebel force trying to oust Assad.

“The Russians certainly have their ideas; we have ours; other players have theirs,” said Susan Rice, Obama’s national security adviser, noting that discussions this weekend among foreign ministers in Vienna will set the stage for the G-20 talks.

Asked whether the United States could accept a plan that allows Assad to remain in power, Rice said: “We do not believe so. Our strong view is that Assad has lost all legitimacy; that the fact that he has been directly responsible for the deaths of so many of his own people means that a transition ultimately will have to result in a legitimate government coming to power in the context of a transition. And it’s very hard to envision how that could be accomplished with Assad still in power.”

The gathering in Antalya, a Mediterranean resort city under an extreme security lockdown in southern Turkey, represents high stakes for Obama. The president is running out of time to make headway against the Islamic State militants who have exploited the bloody Syrian civil war to win control of broad expanses of territory in Syria and Iraq.

Putin’s aggressive moves in trying to shape events inside Syria have complicated Obama’s challenge. The Russian leader enters the summit at a stronger position than a year ago, when he went home early from the G-20 meetings in Brisbane, Australia, after being criticized over Russia’s occupation of the Crimea region in Ukraine.

“What a difference a year makes. Mr. Putin will be far from isolated,” said Heather Conley, a Eurasia expert at the Center for Strategic and International Studies. “Ukraine will not be discussed, and Russia now plays an increasing role in how to resolve the political transition in Syria.”

The White House has left open the possibility of reaching common ground with Putin, and the downing of a Russian airliner over the Sinai Peninsula two weeks ago could change the Kremlin’s calculus. Investigators believe the plane may have been brought down by a bomb planted on board by the Islamic State.

Aides said earlier this week that Obama would participate in a meeting with key European allies to discuss the security situation in both Syria and Ukraine, sending a unified signal to Putin.

“I expect Putin will be on the defensive,” said Derek Chollet, who served as assistant secretary of defense for international security affairs from 2012 to 2014. “I can’t think of a single country that’s pleased with his intervention in Syria. It’s a testing moment for him to see how he responds. The onus is on him to deliver Assad.”

Yet Putin is not the only one whose goals in the region differ from the Obama administration’s.

For Turkish President Recep Tayyip Erdogan, the summit is an opportunity for prominence on the world stage, just weeks after his party consolidated its control by winning a surprising majority in national elections. His bid to exclude Turkish journalists from the summit has prompted protests from human rights groups.

Obama will meet with Erdogan on Sunday.

In speeches and interviews this week, Erdogan repeated his call for a U.S.-protected safe zone in Syria, along at least part of its border with Turkey, where anti-Assad fighters could regroup and be resupplied and refugees could return to a protected area of their own country. Anticipation of such a zone was part of the reason Turkey agreed last summer to allow U.S. strikes against the Islamic State to be flown out of its Incirlik Air Base, and is now participating in the strikes with its own jets.

The administration has long rejected establishing such a zone, because it has not wanted to become directly involved in the civil war and because the Pentagon argued that it would take too many resources away from airstrikes against the Islamic State elsewhere in the country.

“We’re all very realistic that for the conflict in Syria to be resolved, which it must, it’s going to require all the relevant players to come to a common understanding,” Rice said.