World leaders pledged to seize on the Paris attacks to deepen their involvement in what looked Monday to be turning into more of a global campaign against the growing threat of Islamic State.

President Barack Obama vowed to intensify all elements of his administration’s strategy against Islamic State, calling the terrorist attacks in Paris a “terrible and sickening setback” in the fight against extremism.

He pointed to recent U.S. raids, airstrikes and ground deployments as the likely pattern for coming action. But Mr. Obama said he would reject proposals calling for large deployments of U.S. troops.

“There will be an intensification of the strategy that we’ve put forward, but the strategy that we’ve put forward is ultimately the strategy that’s going to work,” Mr. Obama said at a news conference following a two-day Group of 20 summit of leading nations. “It’s going to take time.”

As Mr. Obama spoke in Antalya, Turkey, French President François Hollande, leading a nation in mourning, vowed to take more strenuous action, telling lawmakers that France was “at war against jihadist terrorism that threatens the whole world.”

He outlined domestic steps, including enhanced surveillance, to put France on a security-focused footing, and said he would reach out to the United Nations and European neighbors for support.

He also signaled a shift in foreign policy by proposing a single alliance with Russia and the U.S. to combat Islamic State, the militant group Mr. Hollande blamed for the massacre.

Russian President Vladimir Putin said at the G-20 summit that the attacks proved his case for a deeper international partnership, including Russia, in the fight against Islamic State.

“It seems to me that everyone is coming around to the realization that we can wage an effective fight only together,” Mr. Putin said after leaders at the summit declared the fight “a major priority for all of our countries” and pledged cooperation.

The U.K. announced plans Monday to boost its counterterrorism resources, including hiring 1,900 more intelligence staff, and to step up its aviation security to counter the threat posed by Islamic State.

Even German officials signaled they were considering greater military involvement, despite public opposition to abandoning the country’s largely pacifist foreign policy. Two leading lawmakers suggested that the country should take part in military operations if carried out by a broader coalition.

The vows were accompanied by an intensification of action against Islamic State. French warplanes bombed Islamic State sites, using a new intelligence-sharing pact with the U.S. American aircraft for the first time targeted Islamic State oil tankers, bombing more than 100 as part of an escalating campaign against the group’s lucrative petroleum dealings.

However, the extent of the new effort depends on how many more military resources, such as troops, world leaders are willing to commit.

Despite the emotional aftermath of the Paris attacks, Mr. Obama again ruled out large-scale U.S. deployments. “Maybe part of the reason is because every few months I go to Walter Reed,” he said, referring to the medical facility in Maryland. “And I see a 25-year-old kid that is paralyzed or has lost his limbs.”

He also derided domestic critics who charge his administration underestimated the ability and reach of Islamic State’s extremists. If “folks want to pop off and have opinions about what they think they would do, present a specific plan,” he said.

But, as in other countries, political pressures are building beneath Mr. Obama.

While few public officials have advocated major deployments, Sens. Dianne Feinstein (D., Calif.) and Lindsey Graham (R., S.C.), a Republican presidential hopeful, suggested in interviews that a new international coalition could come together in Syria if France formally requested help under the North Atlantic Treaty Organization’s mutual defense provisions.

“There’s only one way we are going to diminish them and that is by taking them out, because they are growing,” said Mrs. Feinstein. “They are in more than a dozen countries now.”

However, NATO action appears unlikely. French and U.S. officials said both countries oppose NATO’s involvement in the Syria war, arguing the current U.S.-led coalition is a better mechanism for prosecuting the war.

A French official also said Paris was keenly aware that a so-called Article 5 declaration was becoming a political issue in Washington, and feared a declaration would be used by hawks to pressure the Obama administration to expand the war in a way that would prove unpopular in Europe.

Mr. Obama is planning to attend two other international summits, both in Asia, where the subject of the Paris attacks is likely to come up. At home, where fears of a similar attack are rising, pressure is rising on him.

Local and national security officials began increasing security in Washington in response to videotaped threats by Islamic State, and as top intelligence officials warned Monday that more extremist attacks are likely.

“I certainly would not consider it a one-off event,” Central Intelligence Agency Director John Brennan said of the Paris attacks, adding that more plots probably were “in the pipeline.”

Another intelligence official said security professionals must develop new ways to detect and thwart terror plots because militants are better able to evade surveillance.

“Clearly the adversary has gone and is going to school against our capabilities,” said Robert Cardillo, director of the National Geospatial-Intelligence Agency, which gathers information for policy makers, the military, and others. “They are learning…to plan and to execute these heinous attacks in ways that stay under our ability to detect them.”

Despite the emerging consensus that more must be done, divisions remain among world leaders on a coordinated path forward, particularly the future of Syrian ruler Bashar al-Assad.

Messrs. Obama and Putin met informally Sunday to try to coordinate the fight against Islamic State in Syria more closely. There was no breakthrough. Russia continues to carry out an air campaign in Syria not only targeting Islamic State but also other opponents of the Assad regime, U.S. officials maintain.

A senior administration official cautioned that nothing of substance in the U.S.-Russian relations has changed since the Paris attacks, and that there was nothing on the table now to suggest that the two countries could join forces in Syria in a meaningful way.

But some officials said cooperation could be beneficial, despite major differences. “I have had several conversations with one of my Russian counterparts over the past several weeks about ways to strengthened U.S.-Russian counterterrorism cooperation, specifically on the ISIL threat,” Mr. Brennan said in his speech Monday, using another name for Islamic State.

The U.S. and Russia are in talks on a political resolution to the Syrian conflict. On Saturday, they and other nations agreed on broad outlines of a transition, but still disagree on Mr. Assad, with Russia saying he should stay, while the U.S. and others say he must eventually step down.

The White House hopes a political agreement in Syria would pave the way for U.S.-Russia military cooperation against Islamic State.

Mr. Obama said he sees the chances of success as greater than at any time in the past because all parties, including Russia and Iran, are at the negotiating table.