WASHINGTON, Sept 11 - A videotape televised on Sunday purportedly from a U.S.-born member of al Qaeda threatened Los Angeles and Melbourne, Australia, on the fourth anniversary of the September 11 attacks. ADVERTISEMENT ABC News said it had received the video in Pakistan on Saturday. It reported that the masked speaker appears to be Adam Gadahn, from southern California, who threatens attacks on the two cities, "Allah willing," and warns that the attackers will show no compassion. "Yesterday, London and Madrid. Tomorrow, Los Angeles and Melbourne," he said. "We love peace, but peace on our terms" the speaker said. Russ Knocke, a spokesman for the Department of Homeland Security, said U.S. authorities were trying to verify the tape but "there's no specific intelligence to verify or substantiate" the claims and threats in it. "We are certainly aware that al Qaeda seeks to strike the United States and our interests abroad," he said. "We maintain a strong and vigilant security posture." Gadahn is believed to have been the young American who appeared in another threatening tape about year ago. ABC said the young man apparently converted to Islam at an Orange County, California, mosque as a teen-ager. Los Angeles Police Chief William Bratton told ABC News his city had strong security measures already in place but was not ratcheting them up because of this particular tape. "We have a very robust counterterrorism operation here in LA, similar to New York and Washington because we are always a prime target," Bratton said. He added that Gadahn seemed to be "a mouthpiece, a spokesperson, not an operative."