Osama bin Laden (L) sits with his deputy Ayman al-Zawahri in this file photo image supplied by the Dawn newspaper on November 10, 2001. Suddenly, the faces and voices of bin Laden and Zawahri are everywhere, in a stream of video and audio messages broadcast to the world. [Reuters]

Suddenly, the faces and voices of al Qaeda leader Osama bin Laden and his deputy Ayman al-Zawahri are everywhere, in a stream of video and audio messages broadcast to the world.

In the past month alone, six new tapes from the two have reached an international audience. Excerpts of Zawahri's latest message were broadcast on Al Jazeera television on Thursday, a day before the first anniversary of the London bombings.

But U.S. officials and terrorism experts are wary of concluding that the spate of messages means another major attack is imminent.

Instead, they believe a complicated mix of factors is behind the outpouring: a desire to show that al Qaeda is still potent; a new sophistication in the use of propaganda, and finally, sheer coincidence as several different messages have all surfaced within a short time span.

US officials and terrorism experts said they take al Qaeda's threats seriously. The two men are believed to be hiding somewhere in the hostile, tribal border area between Pakistan and Afghanistan.

Bin Laden was not heard from for a year prior to January 2006. But he and Zawahri have now issued 11 audio and video tapes this year, the highest frequency recorded since the September 11 attacks, analysts say.

"They are trying to prove that the movement's not dead," said Kenneth Katzman, a terrorism analyst at the Congressional Research Service, the in-house think tank of Congress.

The two leaders may have felt they had to respond quickly to last month's US military success in killing Abu Musab al-Zarqawi, the leader of al Qaeda in Iraq.

A failed US attempt to kill Zawahri in January and possible greater ease of movement for al Qaeda leaders in Pakistan's northwest frontier region might have also contributed to the higher volume of tapes, Katzman said.

Ben Venzke, head of intelligence company IntelCenter whose clients include the US government, said the back-to-back timing of the messages did not mean they were actually designed to produce a threatening crescendo.

"Does this correlate to any kind of future attacks? I think it doesn't lend itself to an easy yes or no," he said, although certain elements of past messages such as references to US territory could indicate an increased likelihood of a future attack.

Venzke saw some of the tapes as a quick al Qaeda response to major events, such as the death of Zarqawi. Others were more general commentaries on current events which were issued when they were ready. Still others were anniversary features issued to mark the date of a past attack.

Growing Media Savvy

In part, experts traced the recent wave of messages to al Qaeda's increasing media savvy and better logistics.

"It's a result of their ongoing propaganda efforts which have become even more sophisticated," a US counterterrorism official said. "It demonstrates that they've greased the wheels. They've gotten better at this with time."

Coupled with its growing production expertise, al Qaeda leaders have increasingly felt compelled to reassure followers after setbacks while claiming credit for events that seem favorable to their cause.

"Bin Laden and Zawahri are trying to piggy-back on events they consider favorable, such as the Taliban resurgence, the upsurge of Islamic militants in Pakistan, the takeover by the Islamic Courts Union in Somalia. By coming out with this many videos, they are trying to give the impression that 'this is because of us,"' Katzman said.

Gen. Russ Howard, an Army terrorism expert who retired last year, said al Qaeda might be trying to debunk US assertions that the organization was losing central control of its supporters to local or "homegrown" Islamic militants who operate independently.

"This may be a bit of propaganda asserting that there is some type of central control -- that maybe we have this all wrong," Howard said. "It may be a way of telling those franchise groups or wannabe groups that al Qaeda is still in the game, even despite the killing of Zarqawi."