Osama Bin Laden may be dead - but experts believe the NSA links are helping al-Qaeda evolve [Reuters].

Al-Qaida and other terrorist groups have begun changing their means of communication following Edward Snowden's disclosures about NSA's Prism surveillance programme, according to US intelligence experts.

The leaks have given extremist groups more information about the surveillance programmes of the US intelligence agencies, helping them to create new techniques to evade detection.

According to two American intelligence officials who spoke on condition of anonymity, members of every major militant group have started changing their communication methods after the fallout of the revelations. The authorities admitted this has presented a huge challenge to the intelligence community.

A lawmaker who is familiar with the matter told the news agency that al-Qaida's Yemeni offshoot, al-Qaida in the Arabian Peninsula, was among the first groups to change its mode of communication with its operatives.

House Intelligence Committee chairman Mike Rogers said there are "changes we can already see being made by the folks who wish to do us harm, and our allies harm" and added that the leaks might "make it harder to track bad guys trying to harm U.S. citizens in the United States."

Earlier, NSA's director Keith Alexander said the exposé about surveillance programmes had done "irreversible and significant damage" to the US and added the leaks "will hurt us and allies."

He added during a congressional hearing that more than 50 terror plots have been foiled with the help of surveillance programmes.

Snowden, who is currently holed up in an airport in the Russian capital, has already been charged with unauthorised communication of national defence information and wilful communication of classified communications intelligence information to an unauthorised person.

Washington authorities have long been arguing the leaks have wreaked a severe damage to national security and attempting to bring Snowden to the US to try him.