WASHINGTON — President Obama told the nation yesterday that he is pleased by his administration's progress over its first 100 days but warned that much work remained to correct the U.S. economy and protect the country against such threats as nuclear proliferation and pandemic flu.

"So we are off to a good start, but it is just a start," Obama said to begin a prime-time news conference. "I am proud of what we have achieved, but I am not content."

Obama spent his 100th day in office in much the same way he spent the previous 99 — in the public eye. He began the day welcoming Pennsylvania Sen. Arlen Specter into the Democratic Party, bringing it closer to a filibuster-proof 60 votes in the Senate, and ended it with the news conference.

In between, Obama traveled to Missouri, a state he narrowly lost to Sen. John McCain, R-Ariz., in last year's election. At a town hall meeting in Arnold, a suburb of St. Louis, Obama said, "We have begun to pick ourselves up and dust ourselves off, and we've begun the work of remaking America."

"I'm pleased with the progress we've made, but I'm not satisfied," Obama told a cheering crowd at Fox Senior High School. "I'm confident in the future, but I'm not content with the present."

Obama used the 100-day mark, which some administration officials had dismissed as a false benchmark of progress, to capitalize on the public's increased attention on the milestone to promote his policies. A Washington Post-ABC News poll conducted this month found that two-thirds of Americans approve of Obama's performance, a far higher rating compared with his predecessor at the same point in his presidency.

Obama campaigned last year on a promise to change Washington and rescue an economy in deep recession — a legacy, he said at the town-hall meeting, of an "era of selfishness and greed."

But in his first weeks in office, Obama found himself embroiled in partisan politics that threatened his $787 billion stimulus bill, the centerpiece of his economic recovery efforts.

Since pulling off a razor-thin victory on that legislation, Obama has appealed to the public on behalf of his packed agenda, which he characterized in the town hall forum as appropriate given the size of the challenges the country faces. Telling the audience that "my campaign wasn't born in Washington," Obama noted that, as a result of his stimulus plan, Missouri's largest wind farm would add 2,500 jobs.

Obama outlined what he considered his early achievements, including setting a timetable to withdraw U.S. combat troops from Iraq; closing the military brig at Guantanamo Bay, Cuba; cutting taxes for most Americans; and helping keep cash-strapped mortgage holders in their homes.

He said his administration is "clear(ing) away the wreckage" of the subprime mortgage crisis threatening the U.S. banking system and has set its sights on building an economy whose growth does not rely on borrowing.

"I want to warn you, there will be setbacks," Obama said. "I will continue to measure my progress by the progress that you see in your own lives. And I believe that years from now we are going to be able to look back at this time as the moment when the American people once again came together to reclaim their future."