U.S. efforts to counter al Qaeda recruiting online are bearing fruit, and the terrorist group is urging its members not to believe what they read on the Web, according to the State Department.

“We can tell that our efforts are starting to have an impact because we monitor the extremists venting their frustration and asking their supporters not to believe everything they read on the Internet,” Secretary of State Hillary Rodham Clinton told a U.S. Special Operations Command conference in Tampa, Fla., on Wednesday evening.

Mrs. Clinton highlighted a recent effort in which diplomats answered anal Qaeda recruiting campaign. Al Qaeda’s affiliate in Yemen recently “began an advertising campaign on key tribal websites, bragging about killing Americans,” she said.

The campaign involved al Qaeda supporters posting pictures of coffins draped in the U.S. flag on Arabic-language discussion forums used by many young men from Yemen’s fractious tribes.

“Within 48 hours, our team plastered the same sites with altered versions of the ads that showed the toll al Qaeda attacks have taken on the Yemeni people,” Mrs. Clinton said.

The counterpostings showed coffins draped in the Yemeni flag and Arabic messages about the human cost of terrorism in the country, officials said later.

The postings were clear that they came from the U.S. government, State Department spokeswoman Victoria Nuland said Thursday.

The posts were part of a new interagency effort in the State Department- the Center for Strategic Counterterrorism Communications, which President Obama established in September by executive order.

The center has a $6 million annual budget, and its 40 staff come from the State Department, the Pentagon and the U.S. intelligence community, a U.S. official told reporters Thursday.

The center’s 20-strong Digital Outreach Team focuses on Arabic, Urdu and Somali language Internet forums where angry young Muslims provide opportunities for al Qaeda recruiters.

“In the digital world, there’s a whole middle ground where [al Qaedarecruiters] are operating and they seek to push radical views and seek to push young men to go from being, say, anti-American to being terrorists,” the official said.

On these forums, al Qaeda operatives “have people who write in, and we have people who write in against what they do. … They put links to YouTube videos, we put links to YouTube videos. Sometimes there’s a one-to-one correlation, where we’re responding to something they’ve done, as in [this] case,” the official said.

The team’s small size does not matter because it is focused on a small audience, said William McCants, a former State Departmentcounterterrorism adviser.

“They aren’t focused on the whole region,” said Mr. McCants, who now works on counterterrorism at the nonprofit Center for Naval Analyses. “They are focused on a very, very specific group of people, the same audience al Qaeda [recruiters] are going after.”

*Guy Taylor contributed to this report.