Five central leaders of Khorasan, a faction of the Al Qaeda-offshoot Nusra Front in Syria that has come in for particular targeting by US airstrikes, have sworn allegiance to Islamic State (ISIS) leader and "caliph" Abu Bakr al-Baghdadi, according to jihadist websites.

The leaders have pledged to act as soldiers for the "caliphate," namely the Islamic state ISIS has established in the areas of Syria and Iraq it has brutally conquered.

Likewise, they have sworn to conduct armed jihad holy war against "tyrants, their collaborators, heads of apostasy, the United States and its satellite states."

America intelligence has listed Khorasan as a danger even greater than Al Qaeda, saying it has threatened terrorist attacks in America and Europe, and specifically threatened to target international commercial airlines.

In response, America has gone after the group in its airstrike campaign, announcing last Wednesday that it had taken out the head of the group, Musin al-Fadhli.

In fact, Khorasan is not an independent organization, but rather rather a branch of Nusra Front, which as noted is linked to Al Qaeda - as was ISIS before it branched off into its own group.

Khorasan is reported to be made up of veteran Al-Qaeda fighters from Afghanistan and Pakistan who have traveled to Syria to join in the jihad against Syrian President Bashar Assad.

In targeting the group, the American administration has been confronted by reports its strikes are killing Syrian civilians as well as terrorists. It responded last week by saying its policy to only conduct strikes when there is "near certainty" of no civilian deaths does not apply to the current US airstrikes in Iraq and Syria.

The statement, justifying civilian casualties because they occur in an area of "active hostilities," may strike some as hypocritical given America's harsh criticism of Israel's defensive operation in Gaza targeting Hamas terrorists embedded in civilian populations.