ANTALYA, Turkey -- The deadly Paris attacks are pressuring President Barack Obama to confront Islamic State militants more aggressively than he has been willing to consider in the past, opening a difficult deliberation for a leader who has tried to build a legacy on ending America's wars, not extending them.
On Sunday, Obama pledged to "redouble our efforts" to fight international terrorism after the brutal bombings and shootings that killed at least 129 people in the French capital.
Obama mentioned fortifying borders and continuing diplomatic talks, but that strategy is likely to evolve and get tougher as world leaders gather in Turkey for the annual Group of 20 economic summit -- an event transformed by the terrorist strikes into an emergency strategy session on combating Islamic State.
After a meeting with Turkish President Recep Tayyip Erdogan here Sunday morning, Obama condemned the mass killings as "an attack on the civilized world" and pledged to "stand in solidarity" with French authorities as they hunt down the perpetrators.
The attacks have created a new level of anxiety among European and Middle Eastern leaders as they watch Islamic State's ambitions extend beyond Iraq and Syria to target enemies internationally.
The Paris attacks came just a few days after the bombing of a Shiite Muslim district of Beirut and two weeks after the sudden crash of a Russian airliner. Islamic State has claimed responsibility for all three events, which targeted elements that have been fighting Islamic State on what it views as its home turf.
World leaders are looking again to the U.S. for leadership in the campaign to defeat the militant group. Erdogan is just one of the regional partners who wants the U.S. to send in more ground troops and special-operations forces to help coordinate airstrikes.
Others are pushing for a "no-fly" zone to stop air operations by the government of Syrian President Bashar Assad. Supporters of the move say it would protect Syrian civilians and demonstrate that the West remains committed to defeating the Syrian dictator, thereby undercutting a powerful Islamic State recruiting narrative.
Obama has been resistant to the idea of sinking more deeply into the Syrian civil war, but there are signs that could be changing. Defense Secretary Ashton Carter vowed to his French counterpart that the U.S. would take "additional steps" to respond to the Paris attacks.
But after a morning of meetings, White House officials also made clear that they plan to prod coalition members to do more of what they had already pledged to do within the context of the existing counter-Islamic State strategy. The administration expects "some of our allies, including France, to intensify their efforts" and have more resources to put toward the coalition strategy, Deputy National Security Adviser Ben Rhodes told reporters in Antalya on Sunday.
With some intelligence help from the U.S., French warplanes hit Islamic State targets in Syria on Sunday night, the start of what the French government promises will be a stepped-up military campaign.
Obama still doesn't think that more U.S. troops are the answer to the problem, Rhodes said. Instead, the situation calls for a "more sustainable opposition force on the ground in Syria and partners in Iraq," he said.
With an invasion force off the table, the other options are mainly to continue bombing and targeting terrorist leaders while waiting for the Kurds and Syrian forces to get strong enough to tackle Islamic State, also known as ISIS and ISIL.
It was not immediately clear what additional steps Carter meant, but they probably would include intelligence-sharing and closer coordination, such as the assistance offered in Sunday's retaliatory strike by French warplanes.
Frederic C. Hof, a former special adviser to Obama on Syria who now is a senior fellow at the Atlantic Council, a think tank in Washington, said the U.S. may need to consider mounting a ground offensive with Western European support to go after Islamic State in Syria. He said the militants cannot be defeated by only an air campaign.
"These horrific attacks and ISIS' claim of responsibility suggest that the administration may have to go back to the drawing board on its assumption that the battle against this organization must, by definition, be a long one," he said. "ISIS in its Syrian headquarters needs to be taken down, and quickly."
Friday's attacks also "raise hard questions for us and France about how this went undiscovered," said Rep. Adam Schiff, D-Calif., the top Democrat on the House Intelligence Committee, in a phone interview Saturday after a briefing by U.S. intelligence officials. He added that the intelligence agencies are reviewing what information they had leading up to the attack in order to see whether any signs were missed.
If the Paris attacks turn out to have been directed by Islamic State's central leadership, it would mark a "big change" in the group's strategy, said William McCants, an expert on Islamic extremism at the Brookings Institution.
Islamic State has focused on state-building in Iraq and Syria in the last year and "has not dedicated a lot of resources to external operations," McCants said in an interview. Most Islamic State attacks outside the region up until now have been carried out by affiliated groups or individuals who were inspired to act by Islamic State propaganda.
"The calculation has been, by the U.S. and other governments, that as long as Islamic State focuses on state-building in Iraq, it is enough to contain them," McCants said. "But if Islamic State has begun successful large-scale external attacks, I think it will increase the popular demand for much larger-scale interventions."
Much of the pressure could come from European allies.
"France has declared this an act of war," said James Jeffrey, a former ambassador in both Turkey and Iraq and now an analyst with the Washington Institute for Near East Policy. "I do not think (French President Francois) Hollande's response will be to continue his minor air force contribution to Obama's very cautious limited containment campaign."
"Hollande," he added, "will not just go along with Obama's game plan."
Obama heard from other world leaders Sunday as European and Middle East heads of state gathered for a day of meetings. He met with Saudi King Salman, an emergency get-together announced after the Paris attacks.
On Monday he will hold a previously scheduled meeting with European allies to talk about fighting Islamic State as well as dealing with Russian President Vladimir Putin's moves into Syria and Ukraine.
White House officials counted it a minor step forward that Obama met for half an hour Sunday with Putin in a pull-aside chat at the G-20. The two "agreed on the need for a Syrian-led and Syrian-owned political transition" that would come after a cease-fire and negotiations mediated by the United Nations, according to a White House written account of the conversation.
Obama said he "welcomes efforts by all nations" to confront Islamic State and noted the importance of Russia's military efforts in Syria focusing on the group, an aide wrote.