WASHINGTON – Airport security officers around the nation have been alerted by federal officials to look out for terrorists practicing to carry explosive components onto aircraft.

The warning came to light Tuesday just hours after a top U.S. military commander said he believes there are al-Qaida cells in the United States.

Also, President Bush presented his most detailed and lengthy argument that al-Qaida in Iraq was essentially the same organization that attacked America on Sept. 11, 2001.

The alert, based on four curious seizures at airports since September, was distributed on July 20 by the Transportation Security Administration to federal air marshals, its own transportation security officers and other law enforcement agencies.

The seizures at airports in San Diego, Milwaukee, Houston and Baltimore included “wires, switches, pipes or tubes, cell phone components and dense clay-like substances,” including block cheese, the bulletin said. “The unusual nature and increase in number of these improvised items raise concern.”

Security officers were urged to keep an eye out for “ordinary items that look like improvised explosive device components.”

“There is no credible, specific threat here,” TSA spokeswoman Ellen Howe said Tuesday. “Don’t panic. We do these things all the time.”

Homeland Security spokesman Russ Knocke described the notice as the latest copy of a routine informational bulletin with TSA workers, airport employees and law enforcement officials.

Earlier Tuesday, Air Force Gen. Victor Renuart, who heads U.S. Northern Command, said the military needs to triple its response teams to counter a growing threat of attack.

“I believe there are cells in the United States, or at least people who aspire to create cells in the United States,” Renuart said. “To assume that there are not those cells is naive and so we have to take that threat seriously.”

As for attacks, he added: “Am I concerned that this will happen this summer? I have to be concerned that it could happen any day.”

Other U.S. officials said last week they did not know of al-Qaida cells in the United States.

Renuart said the military has one brigade-size unit available to respond to nuclear, chemical and biological incidents at home. That number, he said, needs to grow to three. A brigade is about 3,500 troops.

Separately, Bush sought to rebut his critics’ assertion that the Iraqi group was not a threat to American security.

To those who argue that al-Qaida in Iraq is purely an Iraqi phenomenon, Bush said, “That would be news to Osama bin Laden.”

Citing security details he declassified for his speech, Bush said al-Qaida in Iraq was founded not by an Iraqi but long before U.S. forces invaded the country by Jordanian-born Abu Musab al-Zarqawi, who set up operations with terrorist associates in Iraq. Zarqawi formally joined al-Qaida in 2004 and pledged allegiance to al-Qaida chief bin Laden, Bush said.

Bush added: “The merger also gave al-Qaida’s senior leadership, quote, ‘a foothold in Iraq’ to extend its geographic presence, to plot external operations and to tout the centrality of the jihad in Iraq to solicit direct monetary support elsewhere.”

American forces killed al-Zarqawi in 2006; he was replaced, Bush said, by an Egyptian, Abu Ayyub al-Masri, who had “deep and long-standing” ties to senior al-Qaida leadership.

At the time, bin Laden dispatched a senior deputy to aid al-Masri, but the aide was captured and has been sent to the U.S. prison at Guantanamo Bay, Cuba.

“The fact that Osama bin Laden risked sending one of his most valued commanders to Iraq shows the importance he places on the success of al-Qaida’s Iraqi operations,” Bush said.