Hillary Clinton has thrown her weight behind political efforts to rein in US surveillance powers in her most forthright criticism yet of the National Security Agency (NSA).

The former secretary of state, who has hitherto largely stayed out of the debate sparked by leaks from NSA whistleblower Edward Snowden, called on Congress to restore constitutional privacy protections weakened after terrorist attacks on the World Trade Centre.

"We are finally taking stock of the laws that we passed after 9/11," she told Fox News interviewer Greta Van Susteren. "We did all of this in an a hurry because we were worried and scared and now we need to take a step back and figure out how we make sure that the balance between liberty and security is right."

Clinton, who admitted in an earlier CNN interview that she had disagreed with her husband's cautious support for Snowden, defended the government's legal right to carry out some bulk collection of American data but said she now backed efforts in Congress to change the law.

"Laws that were passed after 9/11 gave the executive very broad authority ... what has happened is that people have said, OK, the emergency is over and we want to get back to regular order," she said.

"It's a really difficult balancing act, but you are absolutely right that we need to make some changes to secure that constitutional right to privacy that Americans are due."

The House of Representatives recently passed a version of the USA Freedom Act that seeks to outlaw the bulk collection of American data, although civil liberties campaigners worry that it has been watered down by administration lawyers and are pushing for a tougher version in the Senate.

Clinton was also scathing the of NSA's spying on the leaders of foreign allies such as Germany and was asked whether chancellor Angela Merkel was right to be angry.

"Yes, she should be. That was absolutely uncalled for," replied Clinton. "There is [legitimate counter-terrorism] work that we need to do with the Germans and inside Germany ... that has nothing to do with Angela Merkel's cell phone and that should be off limits."

The Fox News interview, a rare foray by Clinton in hostile political territory, was dominated by familiar questions about her reaction to attacks on US consular staff in Benghazi but the former secretary of state also strayed into more contemporary diplomatic questions during a separate interview on CNN.

Clinton appeared to distance herself from suggestions by current secretary of state John Kerry that the US could seek assistance from Iran in stemming insurgent attacks in Iraq.

"I am not prepared to say that we go in with Iran right now until we have a better idea what we are getting ourselves into," she told CNN.

But Clinton also supported a growing consensus inside Democratic circles that unilateral US military intervention could also be a mistake.

She criticised Iraqi prime minister Maliki, claiming he had "purged the military that we trained ... and forced out some of the most able commanders" and suggested the White House was seeking tougher assurances from him before offering support.

"I think that right now there are those hard negotiations going on," said Clinton. "With respect to air attacks that needs to be part of a larger package and I believe that is part of the intense negotiations that are going on."

She also said it was a mistake for Iraq not to strike an agreement for an ongoing US troop presence after its main military withdrawal.