US Democratic presidential candidate Barack Obama [Images] has said the elusive Al Qaeda [Images] leader Osama bin Laden should be executed if captured alive.

"If he was captured alive, then we would make a decision to bring the full weight of not only US justice, but world justice down on him," 47-year-old Obama said in an interview to CNN, excerpts of which were released by the network on Friday.

He said the actions of bin Laden, the mastermind of the 9/11 terror attacks in the US, have justified capital punishment.

"I am not a cheerleader for death penalty I think it has to be reserved for only the most heinous crimes," Obama said. "But I certainly think plotting and engineering the death of 3,000 Americans justifies such an approach."

"I think this is a big hypothetical, though -- let's catch him first," he was quoted as saying.

In the wide-ranging interview, Obama said the US had "failed to seriously go after Al Qaeda over the last five years because of the distraction of Iraq, I think we are now seeing the consequences of that in Afghanistan."

The Illinois Senator also criticised Afghan President Hamid Karzai for failing to bring order to his country.

"I think the Karzai government has not gotten out of the bunker, and helped to organise Afghanistan the government, judiciary, police forces in ways that would give people confidence," Obama said.

"So there are a lot of problems there. But a big chunk of the issue is that we allowed the Taliban and Al Qaeda to regenerate itself when we had them on the ropes. That was a big mistake, and it's one I'm going to correct when I'm president," Obama said in the interview to be broadcast on Sunday.