By Tabassum Zakaria

NEW YORK (Reuters) - Americans stood in silence and leaders paid their respects on Monday, five years after hijackers crashed airliners into icons of U.S. power in the deadliest attack on American soil.

From the urban landscape of New York to the Pentagon and a Pennsylvania field, tearful ceremonies honored the nearly 3,000 victims of the al Qaeda attacks, stirring traumatic memories.

President George W. Bush, trying to revive public support for the unpopular Iraq war in a congressional election year, urged renewed American resolve and unity in the war on terrorism, which he called a "struggle for civilization."

"Winning this war will require the determined efforts of a unified country. So we must put aside our differences, and work together to meet the test that history has given us," Bush said in a nationally televised address from the Oval Office.

"We face an enemy determined to bring death and suffering into our homes. America did not ask for this war, and every American wishes it were over. So do I. But the war is not over, and it will not be over until either we or the extremists emerge victorious," he said.

Throughout the day, politicians laid wreaths and bagpipes wailed mournfully. Jarring images of Sept. 11 - smoke billowing from the World Trade Center towers, New Yorkers crying in the streets, debris falling -- dominated television broadcasts.

"It's hard to believe that it's been five years. It's always going to seem like yesterday," said Felicia Cappo, who lost a brother in the World Trade Center's south tower.

Bush and his wife, Laura, stood at New York's Fort Pitt firehouse and bowed their heads for two moments of silence, first at 8:46 a.m. EDT (1246 GMT), the moment American Airlines Flight 11 flew into the north tower, and again at 9:03 a.m. EDT (1303 GMT), when the south tower was hit by United Flight 175.

Then they went to Shanksville, Pennsylvania, for a wreath-laying ceremony in the placid countryside where United Flight 93 slammed into the ground after a passenger revolt stopped the plane from attacking Washington. Forty passengers and crew were killed.

Tears welled in Bush's eyes as he hugged and whispered soothing words to family members of those killed at the Pentagon, hit by American Airlines Flight 77, after placing a wreath next to a blackened stone left in a rebuilt wall.

At Ground Zero, where the 110-story twin towers collapsed, New York police and firefighters marched down a ramp into the pit for a flag-waving ceremony on a day of crisp, clear blue skies, eerily similar to Sept. 11 five years ago.

Spouses and partners of victims read out the names of all 2,749 people who died at the World Trade Center.

ELECTION-YEAR DEBATE

The anniversary sharpened an election-year debate over whether America, caught in a vicious conflict in Iraq and apparently no closer to capturing al Qaeda leader Osama bin Laden, is any safer.

Though billed by White House aides as a non-political speech, Bush's address was heavily laden in defense of his Iraq policy. Bush also used his speech to issue a warning to the elusive bin Laden that "no matter how long it takes America will find you and we will bring you to justice."

Despite fierce partisan debate over the war and who can best secure the country, Republicans and Democrats stood together on the U.S. Capitol steps -- just as they did five years ago -- to show common resolve against terrorism.

Underlining the lingering threat from al Qaeda, its deputy leader Ayman al-Zawahri used the anniversary to urge Muslims to hit Western interests and said U.S. allies Israel and the Gulf Arab states would be the next targets.

"We took our eye off the ball," New York Democratic Sen. Hillary Clinton told CBS' "Early Show." "I mean, we diverted resources and attention to Iraq and we didn't finish the job."

Facing the prospect that Democrats could wrest control of the U.S. Congress from his fellow Republicans in the November election, Bush has been pushing his national security credentials as he did during his 2004 reelection campaign.

At the Pentagon, where 184 people were killed on Sept. 11, Vice President Dick Cheney, a lightning rod of criticism over the Iraq war, told a memorial gathering: "We have no intention of ignoring or appeasing history's latest gang of fanatics trying to murder their way to power."

His comment came two weeks after Defense Secretary Donald Rumsfeld angered Democrats by comparing critics of the war to appeasers of Nazi Germany.

(Additional reporting by Kristin Roberts, Will Dunham, Jeremy Pelofsky, David Morgan, Matt Spetalnick and Thomas Ferraro in Washington and Christine Kearney and Dan Trotta in New York)