In excerpts from an interview with CBS' "60 Minutes" program that will air Sunday, Obama was asked whether he underestimated how difficult it would be to fix the U.S. economy when he became president in 2009.

"I always believed that this was a long-term project," the Democratic president told "60 Minutes." He added it would "take time" to reverse "structural problems in our economy that have been building up for two decades."

Obama added in the excerpts, released Friday, that he thought "it was going to take more than two years. It was going to take more than one term. Probably takes more than one president."

When asked whether he thought the U.S. jobless rate might drop to 8 percent by next November's presidential and congressional elections, Obama said: "I think it's possible. But ... I'm not in the job of prognosticating on the economy."

Reducing unemployment is considered key to Obama's re-election chances next year.

The U.S. jobs picture has improved in recent weeks, with the national unemployment rate falling to 8.6 percent from 9 percent. The government also reported this week that the number of Americans filing new claims for unemployment benefits dropped to a nine-month low last week.

But even an 8 percent unemployment rate is considered high - a 4 percent or 5 percent rate is seen as about normal - and if it stays high in coming months, it could complicate Obama's hopes for re-election.

Some independent economists have suggested the national jobless rate is likely to be in the range of 8 percent to 9 percent, leaving millions unemployed over the long run.

A new CBS News poll has Obama's approval rating at 44 percent, with 54 percent of respondents saying he did not deserve a second four-year term.

Only 33 percent gave Obama good marks for his handling of the economy, the lowest of his presidency, according to the CBS poll released Friday.

Republicans are in the process of deciding their party's presidential nominee to challenge Obama. Former Massachusetts Governor Mitt Romney and former House of Representatives Speaker Newt Gingrich are considered the front-runners.