The Islamic State (Isis) has called on Taliban fighters to defect and on wives of al-Qaeda fighters in Syria to leave their husbands in the latest chapter of an unfolding war for jihad leadership that is increasingly fought from the pages of propaganda magazines.

The appeals were contained in the latest issue of IS' slick English-language publication, Dabiq, which the extremist group releases online on a monthly basis to boost its carefully cultivated image.

In an article headlined They are not lawful spouses for one another, IS claims that all militants with other rebel groups involved in the Syrian conflict, including al-Qaeda's official offshoot, the Nusra Front, are to be considered infidels.

The female writer going by the name of Umm Sumayyah al-Muhājirah goes on to say that, according to the jihadi group's draconian interpretation of Sharia law, the fighters' spouses are thus compelled to leave them and join IS instead.

"It's not permissible for you in any case to remain under the same roof with someone who has removed the noose of Islam from his neck," the piece reads, also warning that those who stay will be punished for "fornication".

In another article IS attacks the Afghan Taliban and its leader, Mullah Mohammed Omar, also disputing whether the one-eyed cleric who struck an infamous alliance with Osama Bin Laden is still alive.

Presented as a fatwa, a legal ruling issued by an Islamic authority, the article juxtaposes Omar to IS leader, Abu Bakr al-Baghdadi, claiming the latter is the only rightful claimant to the crown of leader of all Muslims.

It lambasts the Taliban's decades-long fight to turn Afghanistan into an Islamic emirate as un-Islamic for its nationalist aim, as compared to IS' dreams of global domination, and once again urges all Taliban fighters to join IS instead.

The publication of Dabiq's 79-page tenth issue responded of a few days into the launch of the Nusra Front's equivalent propaganda means, Al-Risalah, released online earlier this month.

In a glossy-designed article titled Khilafah one year on the author dismisses the rise the rise of IS as a "false dawn" describing Baghdadi as a false leader and the stooge of Baathist military brass from Saddam Hussein's regime.

The piece accuses the self-styled caliph of having increased hatred towards Islam and Muslims, blood staining his hands with un-Islamic crimes, such as the beheadings of British aid volunteer Alan Henning and the burning of Jordanian pilot Muath Safi Al-Kaseasbeh.

The war of words for the loyalty of jihadi fighters came as IS is involved in bitter fighting against rival groups also on the ground.

In Syria it is currently mainly battling Kurdish and government forces after consolidating its position at the expense of moderate as well as Islamist rebels.

Meanwhile IS affiliates in Afghanistan have reportedly recently gained ground in eastern regions bordering with Pakistan and previously controlled by the Taliban.

Born as al-Qaeda's branch in Iraq, IS eventually became a rival to its former masters due to a rift started by Baghdadi's decision to deploy his forces into Syria.