A mashup of President Obama's answers to questions asked Friday about Syria at a news conference in St. Petersburg. (The Washington Post)

President Obama on Friday acknowledged the American public’s deep reservations about another military engagement in the Middle East but argued that the United States has a moral responsibility to retaliate against Syria’s use of chemical weapons.

Acknowledging that he faces a “heavy lift” in persuading Congress to authorize a military strike in Syria, Obama made a direct appeal to people’s consciences. The president drew parallels to World War II, which the United States was reluctant to enter, and to its efforts helping end genocide in Kosovo in the 1990s, which was unpopular at the time but which Obama said was the right thing to do.

“I was elected to end wars, not start them,” he said. “I’ve spent the last 41 / 2 years doing everything I can to reduce our reliance on military power as a means of meeting our international obligations and protecting the American people. But what I also know is that there are times where we have to make hard choices if we’re gonna stand up for the things that we care about. And I believe that this is one of those times.”

The comments — which came in a news conference at the end of a Group of 20 summit in Russia — marked Obama’s lengthiest explanation to date of his decision to seek congressional authorization for a U.S.-led military strike in Syria. After returning Friday night to Washington, Obama will continue building his case for action, and he said he plans to address the nation Tuesday from the White House.

Obama faces heavy opposition on Capitol Hill. In the House, a majority of members are now on the record as either being against or leaning against a proposed administration resolution on Syria, according to a Washington Post analysis. Members of the House and Senate who have not reviewed classified information on the chemical weapons attacks will be able to do so Monday on Capitol Hill.

As of Sept. 4, lawmakers appear to be tentatively dividing into four camps over military action in Syria.

In the Senate, a majority of senators remain undecided, according to The Post’s analysis. Senate Majority Leader Harry M. Reid (D-Nev.) on Friday formally introduced the resolution to allow limited military strikes against Syria, setting in motion a key test vote that will probably take place Wednesday.

Reid will need 60 votes in that first vote. Democrats expect some liberal opponents of the strike to vote in favor of considering the measure but then oppose it on final passage, when a simple majority will be needed. With Senate Democrats divided, the final vote could be close or even require Vice President Biden to cast a tie-breaking vote.

During Friday’s news conference, Obama said he was not “itching for military action.” But he said an alleged Aug. 21 sarin gas attack on hundreds of civilians — which he said was carried out by Syrian President Bashar al-Assad’s regime — could not be ignored.

“You know, over 1,400 people were gassed,” Obama said. “Over 400 of them were children. This is not something we’ve fabricated.”

He added, “Sometimes the further we get from the horrors of that, the easier it is to rationalize not making tough choices. And I understand that. This is not convenient. This is not something that I think a lot of folks around the world, you know, find an appetizing set of choices. But the question is, do these norms mean something? And if we’re not acting, what does that say?”

Obama arrived in St. Petersburg hoping to demonstrate broad international support for military action. But after several hours of private talks with other world leaders — including a dinner that stretched until 2 a.m. as each leader offered his or her opinion on the Syrian situation — Obama left with no consensus about how the United States should respond.

The White House cited progress enlisting international support, releasing a joint statement signed by 11 of the summit’s participants that calls for a “strong international response to this grave violation of the world’s rules and conscience” and expresses support for U.S. and allied efforts “to reinforce the prohibition on the use of chemical weapons.”

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But in his own news conference, Russian President Vladi­mir Putin, the summit’s host, reiterated his categorical opposition to a U.S. strike and said that of the 20 nations at the summit, only four — Canada, France, Turkey and Saudi Arabia — had offered Obama their full support.

Also Friday, U.N. Secretary General Ban Ki-moon offered his sharpest criticism of potential military action in Syria. While not referring directly to U.S. plans for a military strike, Ban made it clear that such action could be counterproductive.

“I must warn that ill-considered military action could cause serious and tragic consequences, and with an increased threat of further sectarian violence,” said Ban, speaking at the sidelines of the summit. “We should explore ways to avoid further militarization of the conflict and revitalize the search for a political settlement instead.”

Obama met privately with Putin on the sidelines of the summit for about 20 minutes Friday morning. They discussed Syria, but neither convinced the other to change positions.

“We stuck to our guns,” Putin told reporters, although the Russian leader characterized the conversation as a friendly one. Referencing Obama’s case for military action, Putin said, “We do not agree with those arguments, but still we can hear them. We’re trying to find an agreement toward a peaceful settlement of this crisis.”

Putin said that he believed a unilateral strike would be a violation of international law. He argued that the use of force against a sovereign nation is allowed only as a matter of self-defense, and that Syria has not attacked the United States.

Putin also confirmed that Russia’s support for the Assad regime would continue.

“Will we help Syria? Yes, we will,” he said. “We are doing it right now. We are supplying arms. We are providing economic assistance.” He said Russia wants to expand its humanitarian assistance to Syria.

Meanwhile, Secretary of State John F. Kerry arrived in Lithuania to begin a series of meetings with European Union and Arab League leaders aimed at mustering support for a strike on Syria. In meetings across Europe over the weekend, Kerry will seek more public and explicit condemnation of Assad’s government for chemical weapons use and backing for U.S.-led military action against him.

And in Washington, Samantha Power, Obama’s new envoy to the United Nations, made the case for military action before a generally liberal audience at the Center for American Progress, a Washington think tank closely aligned with the Obama administration on most issues.

Power acknowledged that Americans are weary of war and deeply concerned about the prospect of military intervention in Syria. But she said that the United States has “exhausted the alternatives” to military action, having failed to engage the Syrian government diplomatically or persuade Russia to support efforts through the U.N. Security Council.

“The diplomatic process is stalled because one side has just been gassed on a massive scale and the other side so far feels it has gotten away with it,” Power said. “What would words in the form of belated diplomatic condemnation achieve?”

She added: “There is no risk-free door number two that we can choose in this case.”

Obama and his top advisers are scrambling to drum up support on Capitol Hill for the vote to authorize military action. In the news conference, Obama sought to assure Americans weary after a decade of war in Iraq and Afghanistan that a strike in Syria would be limited. He declined to say whether he would go ahead with a strike if Congress voted no. The president said that many of his decisions have been unpopular but that the American people, as well as their elected representatives in Washington, should trust him to offer his best judgment.

“That’s why they elected me,” Obama said. “That’s why they reelected me, even after there were some decisions I made that they disagreed with.”

Aaron Blake, William Branigin, Juliet Eilperin and Ed O’Keefe in Washington, Colum Lynch at the United Nations and Karen DeYoung in Lithuania contributed to this report.