In a US Senate hearing, National Security Agency (NSA) Director Keith Alexander defended the internet and telephone data snooping programmes.

Also, US Secretary of State John Kerry said they showed a "delicate but vital balance" between privacy and security.

The programmes were revealed in newspaper accounts last week.

Meanwhile, the leaker has pledged to fight extradition to the US.

Edward Snowden fled his home in Hawaii for Hong Kong shortly before reports of the top secret programmes were published by the Guardian and Washington Post newspapers last week.

The 29-year-old former CIA and NSA contract worker has admitted giving the newspapers information about NSA programmes that seize vast quantities of data on telephone calls and internet communications from US internet and telephone companies.

US officials have confirmed the programmes exist, with President Barack Obama saying they were closely overseen by Congress and the courts.

Intelligence officials have insisted agents do not listen in on Americans' telephone conversations. And they maintain the internet communications surveillance programme, reportedly code-named Prism, targeted only non-Americans located outside of the US.
'Americans will die'


"It's dozens of terrorist events that these have helped prevent," Gen Alexander said on Wednesday at a hearing of the US Senate intelligence committee.

Gen Alexander said intelligence officials were "trying to be transparent" about the programme, and would brief the Senate intelligence committee behind closed doors before any other information became public.

But the NSA chief said some details would remain classified "because if we tell the terrorists every way that we're going to track them, they will get through and Americans will die".

He added that he would rather be criticised by people who believed he was hiding something "than jeopardise the security of this country".

Senator Susan Collins, a Maine Republican, asked whether it was true or false that the NSA could, as Mr Snowden had claimed, "tap into virtually any American's phone calls or emails" including the US president's.

"False," Gen Alexander responded. "I know of no way to do that."

But Gen Alexander said the agency needed to investigate how Mr Snowden, a relatively low-ranking contract employee, had been able to obtain and leak such sensitive information.

The processes "absolutely need to be looked at", he told lawmakers.

"In the IT arena, in the cyber arena, some of these folks have tremendous skills to operate networks."

Some members of Congress have acknowledged they had been unaware of the scope of the programmes, having skipped previous briefings offered by the intelligence committees.

"I think Congress has really found itself a little bit asleep at the wheel," New York Representative Steve Cohen, a Democrat, said.
'False statements'
Meanwhile, Democratic Senator Ron Wyden, who warned about the programmes last year, has accused Director of National Intelligence James Clapper of misleading a Senate committee in March when he denied that the NSA collected data on millions of Americans.

Republican Justin Amash has called for Mr Clapper to resign, saying Congress could not make informed decisions "when the head of the intelligence community wilfully makes false statements".

The information leaked by Mr Snowden has undoubtedly angered the US government, but so far he has not been charged by the authorities, nor is he the subject of an extradition request.

European leaders have also expressed concerns over the scale of the programmes and have demanded to know whether the rights of EU citizens had been infringed.

Meanwhile, in a news conference alongside UK Foreign Secretary William Hague, Mr Kerry said: "With respect to privacy, freedom and the constitution I think over time, this will withstand scrutiny and people will understand it."