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.
'Americans will die'
European leaders have 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 in Washington DC, Mr Kerry also said the programmes had "prevented some pretty terrible events."
"With respect to privacy, freedom and the Constitution, I think over time this will withstand scrutiny and people will understand it," he said.
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.
Meanwhile, they have defended the programmes as vital national security tools.
"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 programmes 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".
Review the process
Senator Susan Collins, a Maine Republican, asked whether it was true or false that the NSA could, as Mr Snowden has 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 intelligence briefings.
"I think Congress has really found itself a little bit asleep at the wheel," Tennessee Representative Steve Cohen, a Democrat, said.
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 Congressman 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".
What could the NSA possibly access?
| Phone calls | Emails | Social media |
|---|---|---|
|
Where calls are made |
Name, address, birth date from sign-up |
Name, address, birth date from sign-up |
|
Call duration |
Updates, photos and chat conversations |
Updates and photos |
|
Phone numbers of both parties |
Inbox and outbox content |
Details of contacts |
|
Not content of the call |
Chat conversations |
Chat conversations |
|
Unclear if text messages are included |
IP addresses |
Location information from mobile devices |
Original Article
Source: bbc.co.uk
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