NEW YORK (CNN/Money) - Veteran journalist Walter Isaacson said Monday he is resigning as chairman of CNN just 18 months after taking the job.

Isaacson said he is leaving CNN at the end of the spring to become the president and CEO of the Aspen Institute, a leadership and research organization based in Washington.

Walter Isaacson, above, said he is stepping down as the chief of CNN. (Photo: CNN)

The departure of Isaacson, who was editorial director of Time Inc. until taking the CNN job in July 2001, comes as the 24-hours news organization faces growing competition from News Corp.'s Fox News and as media companies gear up for the challenges of covering a possible war in Iraq.

Isaacson, who was also once managing editor of Time magazine, called the Aspen job "an exciting new prospect for what I want to do with the next phase of my life."

"This opportunity was unexpected and unsolicited, and the timing is not perfect, but it is exactly the type of job I have long wanted, so I did not feel I could let it pass," he said in a statement.

The announcement came a day after Steve Case said he will resign as chairman of AOL Time Warner (AOL: Research, Estimates), which owns Time magazine, CNN and CNN/Money, and other properties.

CNN, Isaacson said, has over the last several months kept "faithful to its core mission of being the most authoritative and credible source of information in the world, emphasizing hard journalistic reporting more than opinion shows."

The news organization, he said, "is in a different league than it was a few years ago. Its reporting remains unrivaled as other networks scale back their coverage of the world."

As for a departure date, Isaacson said only that he was leaving in a "few months." Isaacson, 50, will be replaced by Jim Walton, current president and chief operating officer of CNN News Group. Walton will likely have a role in ongoing merger talks between CNN and ABC News, a unit of Walt Disney Co. (DIS: Research, Estimates)

During a conference call with reporters on Monday, Turner Broadcasting CEO Jamie Kellner said that talks with ABC are currently on hold and that Isaacson's resignation will have no effect on a possible merger.

"We are not engaged right now in discussions [with ABC] nor will we be for probably six weeks as our company analyzes whether we think this is a good decision or not," Kellner said. He added that CNN is not considering mergers with the news organizations of other media companies such as CBS, which is owned by Viacom.