The denial came after a Bloomberg News report alleging the NSA used the flaw in OpenSSL to harvest data.
OpenSSL is online-data scrambling software used to protect data such as passwords sent online.
Last year, NSA leaker Edward Snowden claimed the organisation deliberately introduced vulnerabilities to security software.Continue reading the main story 'A mistake'
A German computer programmer has accepted responsibility for the emergence of the Heartbleed bug, according to a report in the Sydney Morning Herald.
Robin Seggelman, a 31 year old from Oelde - 120 miles (193km) north of Frankfurt - is reported to have made the mistake while trying to improve the OpenSSL cryptographic library on 31 December 2011.
"It's tempting to assume that, after the disclosure of the spying activities of the NSA and other agencies, but in this case it was a simple programming error in a new feature, which unfortunately occurred in a security-relevant area," he told Fairfax Media.
"It was not intended at all, especially since I have previously fixed OpenSSL bugs myself, and was trying to contribute to the project."Sydney Morning Herald
The bug, which allows hackers to snatch chunks of data from systems protected by OpenSSL, was revealed by researchers working for Google and a small Finnish security firm, Codenomicon, earlier this month.
OpenSSL is used by roughly two-thirds of all websites and the glitch existed for more than two years, making it one of the most serious internet security flaws to be uncovered in years.
"[The] NSA was not aware of the recently identified vulnerability in OpenSSL, the so-called Heartbleed vulnerability, until it was made public in a private-sector cyber security report," NSA spokeswoman Vanee Vines said in an email, adding that "reports that say otherwise are wrong."
A White House official also denied the US government was aware of the bug.
"Reports that NSA or any other part of the government were aware of the so-called Heartbleed vulnerability before April 2014 are wrong," White House national security spokeswoman Caitlin Hayden said in a statement.
"This administration takes seriously its responsibility to help maintain an open, interoperable, secure and reliable internet," she insisted, adding: "If the federal government, including the intelligence community, had discovered this vulnerability prior to last week, it would have been disclosed to the community responsible for OpenSSL."
Bloomberg, citing two people it said were familiar with the matter, said the NSA secretly made Heartbleed part of its "arsenal", to obtain passwords and other data.
It claimed the agency has more than 1,000 experts devoted to finding such flaws - who found the Heartbleed glitch shortly after its introduction.
The claim has unsettled many.
"If the NSA really knew about Heartbleed, they have some *serious* explaining to do," cryptographer Matthew Green said on Twitter.
The agency was already in the spotlight after months of revelations about its huge data-gathering capabilities.
Documents leaked by former NSA contractor Edward Snowden indicated the organisation was routinely collecting vast amounts of phone and internet data, together with partner intelligence agencies abroad.
President Barack Obama has ordered reforms that would halt government bulk collection of US telephone records, but critics argue this does not go far enough.The US government said users should change passwords to patched services
Separate to its denials regarding the NSA, the US government also said it believes hackers are trying to make use of the flaw.Continue reading the main story Dangerous or not?
Internet security firm Cloudfare has cast doubt on the scale of the danger posed by Heartbleed, saying it has been unable to exploit it to obtain the secret SSL keys that would put people's data at risk.
The US company was one of those given early warning of the bug before Monday's announcement, and had 12 days to carry out tests.
"Note that is not the same as saying it is impossible to use Heartbleed to get private keys," blogged software engineering leader Nick Sullivan.
"However, if it is possible, it is at a minimum very hard."
The news prompted news site The Verge to lead with the headline: "Heartbleed security flaw may not be as dangerous as thought"
But the security firm that sounded the first alert stands by its warning.
"We know what we found," Codenomicon chief executive David Chartier told the BBC.
"Access to memory is a very serious vulnerability and it's great that people are taking quick action to upgrade and remediate the problem.
"If you search on the internet you will find many people have replicated the problem."
The Department of Homeland Security advised the public to change passwords for sites affected by the flaw, once they had confirmed they were secure, although it added that so far no successful attacks had been reported.
Several makers of internet hardware and software also revealed some of their products were affected, including network routers and switches, video conferencing equipment, phone call software, firewalls and applications that let workers remotely access company data.
The US government also said that it was working with other organisations "to determine the potential vulnerabilities to computer systems that control essential systems - like critical infrastructure, user-facing and financial systems".
The bug makes it possible for a knowledgeable hacker to impersonate services and users, and potentially eavesdrop on the data communications between them.
It only exposes 64K of data at a time, but a malicious party could theoretically make repeated grabs until they had the information they wanted. Crucially, an attack would not leave a trace, making it impossible to be sure whether hackers had taken advantage of it.