WASHINGTON — One of the world's biggest and most important video news organizations employs no reporters or anchormen, owns no satellite trucks and doesn't even report the news itself.

In just seven years of existence, YouTube — which has made viral sensations out of cute baby videos and Justin Bieber music videos — has grown into an important source of news, drawing audiences that rival those of traditional TV news networks and creating "a new kind of visual journalism," according to a new study that assesses the site's role as an information provider.

The Pew Research Center's Project for Excellence in Journalism found that YouTube has enabled tens of millions of people worldwide to follow news events such as the Japanese tsunami, Middle East unrest and the killing of Osama bin Laden by creating their own "on demand" news agenda, watching developments unfold where and when they determine. Some news videos remained heavily viewed on YouTube for weeks, long after traditional news sources had moved on to other subjects.

Videos related to the 2011 tsunami and its aftermath were the most heavily viewed of any in YouTube's "news and politics" category tracked by PEJ during a 15-month period starting in January 2011. The 20 most-viewed tsunami videos collectively had 96 million views, said PEJ, a nonprofit group based in Washington.

Unlike a traditional news organization, which produces most of its own material or obtains it from other professional sources, YouTube features news videos that come from all over. Videos shot by TV news organizations (and often posted, without permission, by viewers) appear to be the largest source of the most-viewed material, accounting for just over half of the 260 videos that ranked among the most popular during the 15-month period PEJ examined. But "citizen-produced" videos — shot by eyewitnesses with video cameras or smartphones — accounted for nearly 40 percent of the total. The balance came from political parties and advocacy groups or from unidentified sources.

Russia Today, a seven-year-old TV news organization backed by the Russian government, produced 22 of the 260 most popular, most of them about events surrounding the Russian presidential election. The second-leading source of professional news clips was Fox News, although six of the nine Fox videos that made the list were posted by viewers who were critical of the channel's hosts or guests, PEJ said. Among these was an interview with country singer Hank Williams Jr. on "Fox & Friends" in October, in which Williams compared President Obama to Hitler, and a video of pundit Tucker Carlson from January 2011, in which Carlson asserted that NFL star Michael Vick should get the death penalty for abusing and killing dogs.

To some extent, the popularity of news clips on YouTube isn't surprising, given the site's global reach. YouTube is the third-most-visited site on the Internet, behind only Google (which owns YouTube) and Facebook, according to the research company Netcraft. It gets more than 4 billion video views a day, about a third of which come from the United States, PEJ said. One measure of YouTube's power, the study noted, is that the governments of nations such as China, Bangladesh, Libya, Pakistan and Iran have attempted to block YouTube content from their citizens.

But the study, which was done in consultation with YouTube but independent of it, also noted that "a complex, symbiotic relationship has developed between citizens and news organizations" on YouTube. People create and post their videos, and they post videos produced by journalism professionals. News organizations, in turn, incorporate "citizen" videos into their work, which gets posted by the organization or by viewers.

"It's kind of like a billiard table with the balls going back and forth in different ways," said PEJ Deputy Director Amy Mitchell. "We're not saying that people are going to YouTube instead of traditional news sources, but this is a new kind of interaction and a new way of absorbing and learning about events from around the world."

At the same time, the study noted that there are no clear ethical standards about how to identify the sources of material in YouTube videos, leaving viewers in the dark about who posted a video or where the uploader got the footage in a clip. "All this creates the potential for news to be manufactured, or even falsified, without giving audiences much ability to know who produced" a video, the study said.

While most of the most heavily viewed videos in the "news and politics" category studied by PEJ were about major news events, a few were about events largely overlooked by the rest of the media. The most prominent was the death of Marco Simoncelli, a professional motorcycle racer. A video of the accident that killed him during the Malaysian Grand Prix was the most-viewed news video on YouTube in October.

As popular as news videos are on YouTube, they still don't rival the biggest in the entertainment category. The most-watched video of 2011 was "Friday," the four-minute pop song by 13-year-old Rebecca Black, which was viewed more than 180 million times during the year. The most-watched video of all time is Justin Bieber's "Baby," which has been viewed more than 755 million times in the two years since it was posted, or roughly a million times a day.