In a sharp attack on Pakistan's inability to curb terrorism, Democratic White House nominee Barack Obama clashed with his Republican rival John McCain over the issue and stumped him on Bush administration's ‘failed’ policies on Iraq and economy, edging past the Vietnam war veteran in yet another free-flowing debate.

"We will kill bin Laden. We will crush al-Qaeda, that has to be our biggest national security priority," 47-year-old Obama said during the testy 90-minute debate, clearly showing his determination to act on his promise of hot pursuit of militants in Pakistan, a stand often criticised by McCain and that has raised hackles in Islamabad.

"If we have Osama bin Laden in our sights and the Pakistani government is unable or unwilling to take them out, then I think that we have to act and we will take them out," the Illinois Senator said during the second in a series of three debates between the Presidential hopefuls.

Obama tried to tie his Republican rival to President George W Bush's ‘failed’ policies, while McCain pushed his image as a ‘consistent reformer’ at the townhouse style standoff at Belmont University in Nashville, Tennessee. Responding to a question whether the US should respect Pakistani sovereignty and not pursue terrorists who maintain bases there, Obama said: "It's a terrific question and we have a difficult situation in Pakistan."

Obama, who had outshined 71-year-old McCain in the September 26 debate as well, said he believed the US had to change its policies on Pakistan.

"I do believe that we have to change our policies with Pakistan. We can't cuddle, as we did, a dictator, give him billions of dollars and then he's making peace treaties with the Taliban and militants," Obama said, referring to former Pakistan President Pervez Musharraf.