Snowden leaks: NSA's Keith Alexander to brief Senate
Gen Alexander: "Great harm has already been done by opening this up and the consequence, I believe, is our security has been jeopardised"
The US electronic spying chief has promised to give the entire US Senate a rare classified briefing later about dozens of terror plots he says were thwarted by secret surveillance.
On Wednesday, National Security Agency (NSA) Director Keith Alexander defended the programmes revealed by a former intelligence worker, Edward Snowden.
He said the US was less safe as a result of Mr Snowden's actions.
China says it has "no information to offer" on Mr Snowden's whereabouts.
The leaker, who has pledged to fight extradition to the US, 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.
'Americans will die'
In the unusual classified briefing for all senators later, Gen Alexander is set to describe successful eavesdropping operations that he will say prevented "dozens of terrorist events" both in the US and overseas.
In his first public questioning since last week's revelations, the NSA chief told the Senate Intelligence Committee that intelligence officials were "trying to be transparent" about the programmes and would brief the committee behind closed doors before any other information became public.
But 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.
Gen Alexander rejected Mr Snowden's claim that while working for the NSA he could wiretap any American, including the president, from his laptop.
Gen Alexander added that he would rather be criticised by people who believed he was hiding something "than jeopardise the security of this country".
In the course of Wednesday's hearing, committee chairwoman Dianne Feinstein revealed that phone records were only accessed by the NSA in cases where there was reason to suspect an individual was connected with al-Qaeda or Iran.
She also said the vast majority of records were never accessed and were deleted after five years.
US Secretary of State John Kerry said the programmes showed a "delicate but vital balance" between privacy and security.
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 the US.
Members of Congress are still expressing concern about the sheer scope of the programmes revealed by Mr Snowden's leaks, says the BBC's Paul Adams in Washington.
They also want to know how it was that a 29-year-old civilian contractor was able to gain access to such large quantities of sensitive information.
Some have acknowledged they had been unaware of the scope of the NSA programmes, having skipped previous intelligence briefings.
"I think Congress has really found itself a little bit asleep at the wheel," said Tennessee Representative Steve Cohen, a Democrat.
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