The president's statement came after news that espionage targeted at foreigners includes Google and Facebook use and credit-card transactions.
Obama said monitoring of U.S. citizens is limited to what has been authorized by Congress.
"When it comes to telephone calls, nobody is listening to your telephone calls," he said. "That's not what this program's about. As was indicated, what the intelligence community is doing is looking at phone numbers and durations of calls."
Obama stressed any further surveillance, including monitoring of the content of phone calls, would require authorization by a judge.
Officials stressed that all the spying, foreign and domestic, seeks to target and weed out enemy national security threats and is authorized under U.S. surveillance law, the officials stressed.
The top-secret online spying, code-named PRISM, has been going on for six years, officials said.
Besides Google Inc. and Facebook Inc. it includes customer information taken from Yahoo! Inc., Microsoft Corp., Paltalk instant messaging, AOL Inc., Skype broadband phone service, YouTube LLC and Apple Inc., records first reported by The Washington Post and British newspaper The Guardian indicated.
Washington acknowledged the PRISM spying after the newspapers reported about it online late Thursday.
The newspapers cited an internal 41-slide PowerPoint presentation for the Defense Department's National Security Agency, a giant global spying operation based at Fort Meade, Md., and nicknamed "No Such Agency" because of its ultra-secrecy.
The PowerPoint slides -- which The Guardian said were apparently used to train intelligence operatives on the PRISM spying program's capabilities -- said the program, run since 2007, has provided the NSA and FBI with the contents of emails, stored data, file transfers, live chats, video conferencing, videos, photos and logins.
PRISM lets the agencies get the information "directly from the servers" of the service providers, the slides said.
The NSA established similar relationships with credit-card companies to get purchase information, three former officials told The Wall Street Journal in an online report Thursday night.
The U.S. officials acknowledged the PRISM spying hours after they acknowledged a separate seven-year effort to sweep up records of phone calls made inside the United States and between a domestic U.S. location and anywhere else in the world.
The phone-call spying of call-detail records goes beyond the spying initially reported through Verizon Communications Inc. to include call data from AT&T Inc. and Sprint Nextel Corp., the two other major U.S. phone networks, the Journal said.
The arrangement means whenever most Americans make a call, the NSA gets a record of the caller and recipient location, the number called, the call time and conversation length, the Journal said.
The program, which evolved out of warrantless wiretapping programs begun by the George W. Bush administration after the Sept. 11, 2001, terrorist attacks, is approved by the government's executive, legislative and judicial branches, officials said.
The phone-call spying, reauthorized by the secret U.S. Foreign Intelligence Surveillance Court, is to end July 19 unless it is authorized again, the court's surveillance warrant indicates.
That court, established by 1978's Foreign Intelligence Surveillance Act, oversees government requests for surveillance warrants against suspected foreign intelligence agents inside the United States.
It was not clear if the Internet or credit-card arrangements are continuing or were one-time-only collection efforts, the Journal said.
The major credit-card and phone companies declined to comment.
Verizon's general counsel said in a memo to employees the company takes steps to safeguard its customers' privacy but must turn over information when ordered to by a federal court.
Several Internet companies issued statements strongly denying knowledge of or participation in the program.
Among the statements, Google said: "Google cares deeply about the security of our users' data. We disclose user data to government in accordance with the law, and we review all such requests carefully. From time to time, people allege that we have created a government 'back door' into our systems, but Google does not have a back door for the government to access private user data."
An Apple spokesman said the company had "never heard" of PRISM.
"We do not provide any government agency with direct access to our servers, and any government agency requesting customer data must get a court order," spokesman Steve Dowling said.
National Intelligence Director James Clapper defended the phone-data surveillance program, saying it is governed by a "robust legal regime."
In a separate statement Thursday night he assailed those who leaked the information to news organizations.
"Information collected under this program is among the most important and valuable foreign intelligence information we collect, and is used to protect our nation from a wide variety of threats," Clapper's statement said.
"The unauthorized disclosure of information about this important and entirely legal program is reprehensible and risks important protections for the security of Americans," it said.
Other government agencies declined to comment about the disclosures.
"Everyone should just calm down and understand this isn't anything that is brand new,'' said Senate Majority Leader Harry Reid, D-Nev.
He told reporters the phone-data program "worked to prevent'' terrorist attacks, without offering details.
Senate Intelligence Chairman Dianne Feinstein, D-Calif., said the program was lawful and must be renewed by Congress every three months.
Senate Intelligence Committee member Ron Wyden, D-Ore., said: "When law-abiding Americans call their friends, who they call, when they call and where they call from is private information. Collecting this data about every single phone call that every American makes every day would be a massive invasion of Americans' privacy."