Al-Qaida's No. 2 Ayman al-Zawahri, said Britain's handover of security in southern Iraq shows that insurgents are gaining the upper hand in the country.
"Reports from Iraq point to the increasing power of the mujahideen (holy war fighters) and the deteriorating condition of the Americans," Zawahri told an off-camera interviewer from As-Sahab, al-Qaida's media arm, in a video posted on the Internet on Sunday.
"And the decision of the British to flee is sufficient (proof of this)," he said, sitting beside shelves full of books.
The video, carried by Islamic Web sites, was issued as Britain handed over security to Iraqi forces in the last of four provinces it once patrolled, effectively marking the end of nearly five years of British control of southern Iraq.
"Iraq is the most important of the fields" in which Islamic militants are fighting, Zawahri said on the video, which carried English subtitles.
Zawahri dismissed optimistic remarks made by current U.S. officials.
"Either the American administration is lying outrageously ... or the Americans have not learned anything from four years of war ... as they did not learn from their defeat in Vietnam," he said.
Zawahri said agents might have infiltrated the ranks of the al-Qaida-linked group Islamic State in Iraq to carry out attacks on the innocent and commit other crimes, in order to encourage Sunni groups to ally themselves with U.S.-led forces against al-Qaida. He called for a swift investigation of this.
"If it becomes clear that some group is involved in these crimes, they should be exposed to defeat the plots of the Americans," he said, calling on other insurgent groups to unite with Islamic State in Iraq.
The Web sites carrying the video invited sympathizers to send in questions during the next month for Zawahri to answer in an "open interview."