Amid a Wall Street panic that sent the Dow Jones industrial average plunging more than 400 points, President Barack Obama sought in a White House address to quell public fears over the nation’s recently downgraded credit rating.

“Markets will rise and markets will fall, but this is the United States of America,” Obama said following the first-ever downgrade of U.S. credit by Standard & Poor’s, from AAA to AA+. “And no matter what some agency may say, we’ve always been and always will be a triple-A country.”

Obama also responded to news over the weekend that 30 U.S. troops had been killed by the Taliban in Afghanistan, telling the nation, “We will press on and we will succeed.”

But Obama’s main theme was the economic turbulence that has roiled markets in Asia, Europe and the U.S. in recent weeks, raising the specter of continuing economic hard times. He attributed the S&P downgrade to Washington’s prolonged debt ceiling fight, which brought the government to the brink of default.

He also called on Congress to extend the payroll tax cut and unemployment insurance benefits, as he has recommended: “These aren’t Democratic proposals. These aren’t big-government proposals. These are all ideas that traditionally Republicans have agreed to.”

The president sought to bolster sagging public confidence, which has been buffeted by persistent unemployment, political wrangling and the decadelong Afghan war. Treasury Secretary Timothy Geithner, who faced speculation that he wanted to step down recently, has told the president that he will remain in his post.

Obama acknowledged that the nation’s economic problems are “a legitimate source of concern” but added: “Here’s the good news. Our problems are eminently solvable, and we know what to do to solve them.”

On the heels of the recent debt limit agreement, the S&P’s downgrade should give “a renewed sense of urgency” to the 12-member Congressional committee created by that deal, Obama said. Officially, the bipartisan panel is tasked with finding, by Thanksgiving, $1.5 trillion or more in additional cuts over a decade.

“It’s not a lack of plans or policies that’s the problem here,” Obama said. “It’s a lack of political will in Washington.”