Al-Qaeda's English-language magazine has reappeared months after its founders were killed in a US missile strike, with calls for firebomb campaigns in the United States and chemical weapons attacks.

Defiantly boasting that it was "still publishing America's worst nightmare," Al-Qaeda's affiliate in Yemen released the eighth and ninth issue of the "Inspire" magazine, which first appeared online in July 2010.

The issues eulogized as "martyrs" the two Al-Qaeda figures who helped launch the publication, and then were killed in a US drone strike in September: radical US-born cleric Anwar al-Awlaki and Samir Khan, a Pakistani-American.

"To the disappointment of our enemies, issue nine of Inspire magazine is out against all odds," an unsigned editorial note said. "Inspire is and will be an effective tool regardless of who is in charge of it."

In addition to specials on "Samir Khan: The Face of Joy" and "My Story with Al-Awlaki," the issue provides detailed instructions on how to ignite an "ember bomb" in the United States.

It suggested that the western state of Montana, with its rapid population growth in wooded areas, was a particularly auspicious place to set huge forest fires.

"In America, there are more houses built in the (countryside) than in the cities," read the article signed by "The AQ Chef."

"It is difficult to choose a better place other than in the valleys of Montana where the population increases rapidly."

In the eighth issue, Awlaki speaks from the grave with an article entitled "Targeting the Populations of Countries that are at War with the Muslims" -- explaining that weapons of mass destruction can and should be used.

"The use of poisons or chemical and biological weapons against population centers is allowed and is strongly recommended due to its great effect on the enemy," the article read, listing the US, Britain and France as top targets.

The US-based IntelCenter, commenting on the release of the two issues, said Awlaki's article served as a "clear reminder" that groups linked to Al-Qaeda see such attacks as permissible and greatly important.

"Even though al-Awlaki is no longer alive, his unmatched ability to inspire attacks will continue through his writings and statements, especially with new releases such as this," it said.

The issue also includes an eight-page special on how to remotely detonate a bomb, along with necessary parts and detailed step-by-step photographs, and advice on training with a handgun.

Yet for all the fanfare, both issues are riddled with spelling errors and clumsy English. An ad in the eighth issue asks for "persons who can help the Inspire team with research & translation."