By Peter Baker

The New York Times

STOCKHOLM - President Barack Obama declared Wednesday that the confrontation with Syria over chemical weapons was not a personal test for him but for Congress, the United States and the world as he worked to strengthen support at home and abroad for a punitive strike.

Opening a three-day trip overseas at a delicate moment for his presidency, Obama challenged lawmakers and allies to stand behind his plans for a cruise missile attack on the government of President Bashar Assad in retaliation for what the Obama administration has concluded was a chemical attack that killed 1,400 people in the suburbs of the Syrian capital, Damascus, last month.

"I didn't set a red line," Obama said during a news conference in Stockholm. "The world set a red line."

He added, "My credibility's not on the line. The international community's credibility is on the line. And America and Congress' credibility is on the line."

Obama laid blame for the Aug. 21 attack directly on Assad, whose government is known to have enormous stockpiles of banned chemical munitions including sarin gas, a nerve agent that U.S. intelligence has said was deployed in a rebel-held part of the Damascus suburbs.

U.S. intelligence has not disclosed any evidence that Assad ordered the use of sarin, but the White House has said he remains responsible as the leader of the country and its military. Obama's language Wednesday appeared to go a little further in singling out Assad.

"We believe very strongly with high confidence that in fact chemical weapons were used and that Mr. Assad was the source," the president said.

Obama arrived here Wednesday morning for a one-day stop before heading Thursday to St. Petersburg for a gathering of the Group of 20 nations hosted by President Vladimir Putin of Russia. Putin has opposed any retaliatory attack on Syria, calling such action a violation of international law.

The trip comes as the president and his advisers are trying to convince Congress to pass a measure formally authorizing a punitive strike. Before taking off, Obama won support from Republican leaders in the House.

On Thursday, in addition to G-20 activities, Obama will meet separately with President François Hollande of France, who supports a strike on Syria, President Xi Jinping of China, who does not, and Prime Minister Shinzo Abe of Japan, who has avoided getting in the middle of the dispute.

Obama does not plan to meet with Putin in St. Petersburg, even though that would be typical with a summit host.