Obama, clearly still the reluctant warrior who rose to political prominence on his opposition to the Iraq war, emphasized he favored limited strikes on Syria to deter future chemical weapons attacks - not another costly and protracted conflict.
"This would not be another Iraq or Afghanistan," Obama declared in his weekly radio address, previewing arguments he will make in a nationally televised address on Tuesday.
(Read more: Obama insists Syria 'not another Iraq or Afghanistan')
"I know that the American people are weary after a decade of war, even as the war in Iraq has ended, and the war in Afghanistan is winding down. That's why we're not putting our troops in the middle of somebody else's war," Obama said.
Obama will give interviews on Monday to the three network news anchors, as well as PBS, CNN and Fox News, more evidence of a "full-court press" strategy before pivotal congressional votes on military strikes in Syria.
The interviews will air during each network's Monday evening news broadcast, the White House said.
Lawmakers returning to Washington after a summer break say many of their constituents have told them they do not think the United States should respond militarily to the August chemical weapons attack that Washington blames on Assad's government.
The Obama administration says over 1,400 people were killed by the poison gas, hundreds of them children. A recent Reuters/Ipsos poll said 56 percent of Americans believed the United States should not intervene in Syria; 19 percent backed action.
Obama is seeking congressional approval for a strike, but early vote-count estimates do not look encouraging for the president, with scores of lawmakers still undecided. The Senate is expected to take action next week. The House of Representatives will vote later, but the time is not set.
As the White House cranked up its campaign, CNN showed excerpts on Saturday from the gruesome aftermath of the attack taken from a DVD shown to lawmakers and compiled from publicly available videos on YouTube and other internet sites.
Pressure rises for delay in Europe
Many EU governments have expressed reservations about using military force to punish Assad, now fighting a 2-1/2-year battle against rebels in which more than 100,000 people have died.
In a carefully worded message, the foreign ministers of 28 EU governments stopped short of endorsing possible U.S. and French military action against Syria ahead of the U.N. report.
(Read more: Obama steps up campaign to sway Congress on Syria)
French President Francois Hollande said the report could be made public at the end of next week and he suggested that France might then wish to take the matter to the U.N. Security Council, a step that could further delay any action.
"When the (U.S.) Congress will have voted on Thursday or Friday and when we will have the inspectors' report, likely at the end of the week, a decision will have to be made, including after possibly referring the matter to the United Nations (Security Council)," Hollande said, speaking from the southeastern city of Nice after a meeting with his Lebanese counterpart.
An iFop poll published in Le Figaro on Saturday found that 64 percent of the French opposed any kind of international military intervention in Syria, up 19 percentage points in just one week, with even more - 68 percent - opposing a French intervention in the war-torn country.
A senior Obama administration official suggested on Friday that the White House could wait for a U.N. inspectors' report on chemical arms use in Syria before ordering U.S. naval forces gathered in the Mediterranean to hit Syria.
U.S. Secretary of State John Kerry, who was also in Lithuania, said later that Obama had made no decisions about waiting for the U.N. inspectors and was keeping options open.
Apart from anything else, delay in attacking Syria might help the White House gather more support in Congress and among public opinion.
The senior official told reporters that during Obama's discussions with other G20 leaders in Russia on Friday on the timing of any military response to the Syrian crisis, it was apparent that "a number of countries feel it's important that the U.N. inspectors have time to report back their findings first.
"That's entirely consistent with our timetable," the official said. Final votes in Congress could come after the U.N. report is announced.
Scrambling for votes
Supporters of military action scrambled for votes in Congress. House Democratic leader Nancy Pelosi on Saturday sent her fifth letter to Democratic lawmakers urging them to back Obama, noting that Congress had voted overwhelmingly to condemn Syria's acquisition of weapons of mass destruction a decade ago.
(Read more: Syria focus will fan flames of September volatility)
The influential pro-Israel group AIPAC said it planned a major lobbying effort next week to try to round up support for military action, with about 250 activists in Washington to meet senators and representatives.
But it was unclear whether the effort was working.
Senator Mark Pryor, a member of the president's Democratic Party, who is running for re-election next year, said on Saturday he would not support action against Syria at this time.
Pryor said that before U.S. military action is taken, the administration must prove a compelling national security interest, define the mission and build a "true" coalition of participating allies - criteria he said had not been met.
Outside the White House, about 200 opponents of U.S. action in Syria gathered on Saturday, chanting, "Hands Off Syria" and waving signs that read: "Tell Congress: no war on Syria."
"The American people are tired of war. The government is not," said retired teacher Andra Sufi, 66, of northern Virginia, who was dressed in white and carried a rainbow "Peace" flag.
In New York, tourists entering St. Patrick's Cathedral said they were frightened and depressed by events.
Beth Alberty, a 72-year-old retired museum curator taking part in a Times Square protest against U.S. military action, said she was disappointed in Obama. "This is completely against what he campaigned on in regard to Iraq. And the arguments are very much the same it seems to me in this case. We are creating a reason to go in," she said.
Democratic congressional aides said Obama's planned speech to the nation on Tuesday and briefings that top members of Obama's national security team will give to the entire House on Monday would prove pivotal in the thinking of many lawmakers.
But Republican Representative Justin Amash, who opposes U.S. intervention in Syria, suggested classified briefings would make no difference. "If Americans could read classified docs, they'd be even more against Syria action," he tweeted.