Antalya, Turkey (CNN) President Barack Obama vowed Sunday to renew U.S. efforts against ISIS, saying "the skies have been darkened" in the aftermath of a terrorist killing spree in Paris .

Speaking at the start of the G20 here on the Turkish Riviera, Obama said the two-day summit has assumed new importance as leaders work to develop a response to the massacre.

Underscoring his point, an intense discussion between Obama and Russian President Vladimir Putin was caught on cameras, showing the two men leaning in and gesturing sharply as they made their points.

Later an Obama aide said the two men appeared to reach an agreement on political path forward in Syria -- though U.S. officials said a substantial shift in strategy toward combating the Islamic State was not in the works.

"The killing of innocent people based on a twisted ideology is an attack not just on France, not just on Turkey, it is an attack on the civilized world," Obama said earlier in the day during a meeting with Turkish President Recep Tayyip Erdoğan.

"We will redouble our efforts, working with other members of the coalition, to bring about a peaceful transition in Syria and to eliminate (ISIS) as a force that can create so much pain and suffering for people in Paris," Obama said, adding later that he and Erdoğan discussed ways to fortify Turkey's border with Syria and a strategy for addressing the refugee crisis.

'Act of War'

Afterward, White House Deputy National Security Adviser Ben Rhodes told reporters that the Paris attacks indeed demand more international resources and urgency in the fight against ISIS.

"It is an act of war," he said.

He said airstrikes could be scaled up, as well as targeting of ISIS leadership -- an element Rhodes underscored could have an outsized effect because those are the individuals with the ability direct attacks abroad, as opposed to rank-and-file ISIS fighters.

Speaking on CNN's "State of the Union with Jake Tapper," Rhodes said "there are elements of the strategy that, when they are properly resourced, they work in rolling back ISIL."

"We have to do more of that, and we're going to be able to do that," he said. "We are in an effort to intensify aspects of the strategy that are yielding progress even as we have constantly evaluate how the threat is changing and what more needs to be done to deal with the threat."

In a meeting between U.S. Defense Secretary Ashton Carter and his French counterpart, the men "agreed concrete steps the U.S. and French militaries should take to further intensify our close cooperation in prosecuting a sustained campaign against ISIL," according to a Pentagon statement.

But Rhodes said the White House is still not considering a ground combat role for U.S. forces in Iraq or Syria.

"We don't believe U.S. troops are the answer to the problem," Rhodes said.

Turkey has been pressing the U.S. to help establish and enforce a no-fly-zone or "safe zone" at its border with Syria, and Sunday's meeting between Obama and Erdogan led to speculation that that might be an upcoming change in policy.

But Rhodes was quick to throw cold water on that possibility, saying it would be enormously expensive, and a misallocation of resources at this point.

He also took the opportunity to press Congress to approve a new war powers resolution against ISIS. Obama's proposal has been gathering dust on Capitol Hill for months. Rhodes said approving that plan, which is newer and more tailored than the one the administration has been operating under, would send a signal of the United States' long-term commitment to the fight.

Allies and adversaries

The President is expected to meet allies and adversaries alike, including Putin, whose efforts in Syria have forced the administration to retool its strategy for combating ISIS. A formal meeting between the two men is not scheduled, though Obama was seen engaged in deep conversation with the Russian leader on the sidelines of the summit.

Afterward, a White House official said the two men "agreed on the need for a Syrian-led and Syrian-owned political transition, which would be proceeded by UN-mediated negotiations between the Syrian opposition and regime as well a ceasefire."

That language reflected a shift from earlier White House characterizations of the Russian position, which has castigated Putin for his support of Syrian President Bashar al-Assad.

The official said Obama "welcomed efforts by all nations to confront the terrorist group ISIL and noted the importance of Russia's military efforts in Syria focusing on the group," another change from previous White House charges that Putin was targeting opponents of Assad, rather than ISIS.

Putin's recent entry into the war in Syria -- Russian planes are striking forces opposed to Syrian President Bashar al-Assad -- have driven a further wedge between Moscow and the West.

Originally, Obama appeared to be entering this yearly gathering able to tout recent victories against ISIS, including aiding Kurdish fighters in retaking Sinjar Mountain in Iraq and having apparently taking out "Jihadi John," the high-profile ISIS executioner.

Continued extremist threat

But the deadly Paris attacks, paired with the recent downing of a Russian airliner over Egypt's Sinai Peninsula and bomb attacks in Beirut this week, illustrate the continued extremist threat.

Obama's presence at the G20 brought him significantly closer to the problem. The resort where the meeting is taking place is located 500 miles from the Turkey-Syria border, and authorities arrested 20 suspected ISIS members in the town last week.

Leaders, including Putin, will meet for dinner Sunday to address the refugee flow that has overtaken Europe in recent months, as well as the broader problem of how to rid Iraq and Syria of ISIS. On Monday, Obama will huddle with leaders from the United Kingdom, Germany, France and Italy to further discuss the campaign against the terrorists.

Expectations for breakthrough solutions were low.

"I don't think anybody expects a single outcome that all of a sudden readily resolves all these difficult issues," said Susan Rice, Obama's national security adviser, ahead of the trip. "We're looking to try to use these venues to make incremental progress toward the objective that we all seek."