From Bang to Whimper, ISIS Propaganda Withers Under US Strikes Terror group lowers volume of grim messages, but officials suspect more to come.

 -- As explosions erupted from airstrikes in Iraq and Syria over the past 12 days in the U.S.-led military campaign against terrorists there, the sounds of silence from jihadi propagandists also have intensified.

No major Islamic State of Iraq and Syria (ISIS) video threatening the West or speech by any ISIS leader has been posted online since Sept. 21, with the exception of the latest multi-part installment of "programs" by British hostage John Cantlie, which many sources believe were made prior to the strikes that began the night of Sept. 22.

While ISIS ground forces continue to menace cities and are reportedly advancing on the Kurdish-held town of Kobane on the Syria-Turkey border, the sophisticated and prolific ISIS propaganda machine -- exceptionally skilled at "information operations," according to western intelligence operatives -- has limped along since missiles fired by ships, drones and fighter jets have rained down each night on their positions.

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Its usual stock in trade of videotaped mass executions and pillaging has slowed to a trickle of locally-generated videos showing jihadi fighters promising hell to American warplanes and shopkeepers on bustling Syrian streets griping about bomb damage. One recent video showed a purported ISIS fighter, an English speaker, walking through the rubble of an airstrike.

The other target of the American air campaign is a small but purportedly elite al Qaeda cell in Syria, known to U.S. intelligence as the "Khorasan Group" -- seasoned jihadis who fought together in Afghanistan and Pakistan who are being sheltered by al Qaeda's official Syrian affiliate Jabhat al-Nusra. The Khorasan Group was secretly developing non-metallic bombs to smuggle aboard commercial passenger jets in the West, officials have said, and produced few if any public statements while on Syria since 2011.

It is unclear whether ISIS has suffered losses in its media operation or purposely gone quiet since it released the hour-long pseudo-documentary "Flames Of War" several days before the U.S. stepped up airstrikes. That Sept. 19 video charted ISIS's own version of its meteoric rise from insurgency on the ropes in Iraq three years ago to blitzing eastern Syria and northern Iraq over the summer and declaring an Islamic caliphate.

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The last message from an ISIS leader was an audio speech by the group's spokesman, Abu Muhammad al-Adnani the day before the massive U.S. military onslaught began, in which he urged Muslims to kill Americans and Europeans "in any manner or way."

A new significant ISIS message or video could come at any moment, and some officials said they anticipate more soon, including possibly another hostage beheading like that of two American journalists and a British aid worker since August.

Whether the U.S. succeeded in temporarily disrupting ISIS communications or the group has become quiet publicly in order to maintain security, the relative silence of the past 12 days "may rob them of their momentum or the appearance of momentum," J.M. Berger, co-author of the forthcoming "ISIS: State of Terror," told ABC News.

Berger and others who monitor ISIS social media told ABC News that Twitter suspended more than 1,000 suspect accounts linked to the group during September, including at least 100 foreign fighters.

"A heavy concentration of English speakers got hit," he said, adding that another 400 ISIS-tied accounts were shut down Tuesday.

"We review all reported accounts against our rules, which prohibit unlawful use and violent threats," a Twitter spokesperson, who did not wish to use their name, told ABC News in an email on Thursday about the suspensions.

Twitter officials themselves have been threatened by jihadis in Syria and Iraq for suspending accounts, but the timing of the latest round of eradication -- a week after the U.S. launched its biggest strikes yet -- indicates a continued willingness by the company to silence ISIS fighters and supporters.

Other observers of ISIS social media note that new accounts pop up on Twitter as fast as the company -- which often responds to government requests -- deletes them.

"Their media has been disrupted but it's unclear to what extent," said one official who tracks ISIS.

Jihadi social media, however, has often been exploited for intelligence by western spy agencies -- which ISIS knows, and is part of why their information operations are considered unusually sophisticated, intelligence officials said.

The deaths of al Qaeda leaders have traditionally been reported online in eulogies -- and sometimes faked. An al Qaeda fighter in Syria on Saturday reported the deaths of two senior al Qaeda operatives there in U.S. airstrikes, including a veteran facilitator with a $7 million U.S. bounty.

Muhsin al-Fahdli, 33, was a Kuwaiti senior figure in a bombmaking collective targeting the West who are believed to have been sent to Syria by top al Qaeda leaders in Pakistan several years ago. As a teenager he was so close to Osama Bin Laden he knew in advance about the 9/11 plot and has been considered by western intelligence to be an "influential" member of the group, dubbed "Khorasan" by the U.S. government because most of the 50 or so terrorists had fought together in the Afghanistan-Pakistan region.

The group was targeted by a third of the initial volley of U.S. cruise missile and fighter strikes on Sept. 22 because they were believed to be nearing the "execution phase" of a plot or plots against commercial aviation in either Europe or the U.S. homeland using cutting edge non-metallic improvised explosive devices.

The death of al-Fadhli, who also goes by aliases including Abu Samia and Abu Asmaa', was claimed in tweets by a known al Qaeda operative who had fought in the Afghanistan-Pakistan wars, according to the SITE Intelligence Group, a private firm that monitors jihadi media for news media and government clients.

Reports of the deaths of al-Fadhli and another operative began surfacing on Twitter last week but the U.S. military has been unable to verify it so far, though the military's top general said news likely would first come via social media.

"We scan social media, which is normally the first place you find out, frankly," Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told reporters last week.

"We have no one on the ground. We have no way to tell if he's dead unless they put it on social media," one intelligence official told ABC News.

Some experts urged caution, however, noting that al Qaeda's Syria organization, al-Nusra Front, used twitter to fake the death of another operative earlier this year who was not deceased.

"He could very well be dead but there is no firm evidence yet," said Thomas Joscelyn, a Long War Journal writer and Foundation for the Defense of Democracies expert on al Qaeda, who regularly testifies before congressional committees on counter-terrorism matters.

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