A bomb attack on a bus in the Russian city of Volgograd has killed at least five people, according to officials.

Police say they suspect the bombing, which also injured more than a dozen others, was carried out by a female suicide bomber. 

Russia has long been a target of Islamist terrorism, particularly from the Caucasus, who object to what they see as the Russian occupation of the majority Muslim territories of Dagestan, Ingushetia and Chechnya.

Russian forces fought a two-decade campaign against Chechen separatists, and the Russian mainland has seen a string of terrorist atrocities over the past decades.

But Russian backing for the Syrian regime of Bashar al-Assad has given Islamists another reason to place Russia in their cross hairs.

Assad hails from the Alawite sect of Islam, an offshoot of Shia Islam, and is backed by Shia Islamist power Iran. The rebels, meanwhile, are mostly Sunni Muslims. The bloody civil war between the two sides – now in its third year – has taken on an increasing sectarian flavor, and is being viewed by Islamists on both sides of the conflict as an extension of the age-old struggle between the two largest competing strands of Islam (Sunnism and Shi’ism).

Russia’s staunch support – both militarily and diplomatically – for the Assad regime, has fueled calls by Islamists for a new wave of attacks against it, and has even seen it take the place previously reserved for the U.S. as “public enemy number one” in some Islamist circles.