WASHINGTON (AFP)--Marking his 100th day in office, U.S. President Barack Obama on Wednesday pledged an "unrelenting, unyielding effort" to return the country to prosperity and to confront threats including swine flu and terrorism.

Early in the day he visited the U.S. heartland, telling a town-hall style event in Missouri that while he was "pleased with our progress...I am not satisfied" with the accomplishments in his first four months.

Obama stressed there is a long slog ahead to pull the nation out of its worst recession since the Great Depression in the 1930s.

He brought the same message to a national audience during an evening news conference, in which he covered issues from the upheaval in the U.S. auto industry and legislative bickering on Capitol Hill, to foreign policy touchstones including the need to safeguard Pakistan's nuclear arsenal.

"We still confront threats ranging from terrorism to nuclear proliferation to pandemic flu," he said.

He immediately highlighted the threat of a swine flu pandemic, and vowed that the country would do "whatever it takes" to control the outbreak, which been linked to the death of a child in Texas.

Obama also defended his $3.4 trillion budget for 2010 - approved by the House of Representatives Wednesday and due to be voted on by the Senate later this week - as a budget that "builds on the steps we've taken over the last 100 days to move this economy from recession to recovery and ultimately to prosperity.

"So we are off to a good start, but it is just a start," he said, warning that more pain lies in store.

"Millions of Americans are still without jobs and homes, and more will be lost before this recession is over.

"All of this means you can expect an unrelenting, unyielding effort from this administration to strengthen our prosperity and our security - in the second hundred days, and the third hundred days, and all the days after."

In Missouri, Obama said his administration had begun "remaking America" since taking office Jan. 20 by reversing the contentious policies of George W. Bush's administration and orchestrating a historic $787 billion stimulus bill, among other reforms.

Obama's broad agenda during his debut - seen as a success by most Americans, according to opinion polls - is widely considered one of the fullest plates for any new president in decades, and he openly acknowledged he was stunned by the onslaught of crises facing the administration.

"I am surprised...by the number of critical issues that appear to be coming to a head at the same time," the president said.

"If you could tell me right now that when I walked into this office that the banks were humming, the autos were selling, and that all you had to worry about was Iraq, Afghanistan, North Korea, getting health care passed, figuring out how to deal with energy independence, deal with Iran, and a pandemic flu, I would take that deal."

But in addition to those issues, Obama is tasked with pulling the country out of the steepest economic free-fall since the depression.

In an emergency bid to keep Americans from avoiding foreclosures, a $75 billion fund has been created to help embattled homeowners.

His administration has also intervened to try to help save teetering U.S. auto giants Chrysler and General Motors, and injected billions of dollars into struggling banks and other companies in moves that critics have decried as partial nationalization.

On other domestic fronts, Obama has expanded a state health insurance plan to cover millions of children, broadened unemployment benefits and overturned Bush's restrictions on embryonic stem-cell research.

Abroad, Obama has recast U.S. foreign policy by putting greater emphasis on multilateral diplomacy, reaching out to Muslims and vowing to end decades of enmity with foes Cuba and Iran.

The U.S. president has also made a start on rolling back some of Bush's antiterrorism policies by mandating the closure of Guantanamo Bay prison camp, outlawing torture and setting a timetable for withdrawal from Iraq.

On Wednesday he stressed that the legal rationale used by his predecessor to justify waterboarding against detainees was a "mistake" and that the simulated drowning tactic is "torture."

Obama has also doubled down in Afghanistan and Pakistan to tackle mounting unrest. On Wednesday he said he was "gravely concerned" about the situation in Pakistan, but "confident" the country's nuclear arsenal can be kept out of the hands of Islamist extremists.