New revelations from former security contractor Edward Snowden that US intelligence agencies have access to a vast online tracking tool have come to light, as lawmakers put secret surveillance programs under greater scrutiny.

The Guardian, citing documents from Snowden, published National Security Agency (NSA) training materials for the XKeyscore program, which the newspaper described as the NSA's widest-reaching system that covers "nearly everything a typical user does on the internet".

Intelligence analysts can conduct surveillance through XKeyscore by filling in an on-screen form giving only a "broad justification" for the search and no review by a court or NSA staff, the Guardian said.

Snowden's revelations that US intelligence agencies collected data on phone calls and other communications of Americans and foreign citizens as a tool to fight terrorism have sparked uproar in the United States and abroad.

Intelligence officials say the programs helped thwart terrorist attacks.

"The implication that NSA's collection is arbitrary and unconstrained is false," the agency said in response to the Guardian's new report, calling XKeyscore part of "NSA's lawful foreign signals intelligence collection system."

Lawmakers have called for greater oversight of the vast surveillance system, which expanded rapidly after the September 11 attacks on the United States in 2001.

Intelligence officials were grilled at a US Senate Judiciary Committee hearing on Wednesday (local time) about their data gathering, the lack of transparency and security lapses that let Snowden get away with so much data.

Snowden, who has been charged under the US espionage act and had his passport revoked, left Hong Kong more than a month ago and is stuck in limbo at a Moscow airport while seeking asylum in Russia, which has refused to extradite him.

The latest leak of classified information came as the director of National Intelligence released three declassified documents in the "interest of increased transparency," which explained the bulk collection of phone data - one of the secret surveillance programs revealed by Snowden two months ago.

Much of what is contained in the newly declassified documents already has been divulged in public hearings by intelligence officials as opposition to the government's sweeping data collection programs has been growing.

Snowden also revealed details about the scope of the US PRISM program, which allowed the NSA and FBI to track an individual's web presence.

FBI asked Snowden's father to go to Moscow

What is PRISM? Eavesdropping program used by US intelligence agencies

Eavesdropping program used by US intelligence agencies Allegedly allows the National Security Agency (NSA) and the FBI direct access to servers to track an individual's web presence

Allegedly allows the National Security Agency (NSA) and the FBI direct access to servers to track an individual's web presence Reports Microsoft, Yahoo, Google, Facebook, Apple, AOL, Skype and YouTube among those involved

Reports Microsoft, Yahoo, Google, Facebook, Apple, AOL, Skype and YouTube among those involved Data monitored could include search histories, emails, social media interactions, connection logs, audio and video

Data monitored could include search histories, emails, social media interactions, connection logs, audio and video Washington Post says PRISM program is the most significant contributor to US president Barack Obama's daily briefings, accounting for one in seven intelligence reports

Meanwhile, Snowden's father says the FBI has asked him to visit his son in the Moscow airport.

Lon Snowden told Russian state television he fears his son will not get a fair trial in the US.

He says his son acted valiantly by leaking classified information about US government surveillance.

"My son is a principled young man. He's a man of courage and what he saw he could not live with, it was a moral hazard," he said.

"I know that I raised him to do the right thing, (but) sometimes the right thing means personal sacrifice and that's what he did, so absolutely I am proud of my son."

The interview was broadcast live early in the morning on Rossiya 24, the state-owned 24-hour news channel.

Speaking English, but with a Russian translation, Mr Snowden said the FBI had asked him several weeks ago to fly to Moscow to see his son.

But Mr Snowden, who has had no direct contact with his son, says he would not be an "emotional tool" for the US authorities.

Reuters/BBC