since announcing its “caliphate”, the Syrian Observatory for Human Rights monitoring group said on Sunday.
Nearly 1,800 of them were civilians, including 74 children, it said. They include more than 200 people killed in the Syrian Kurdish town of Kobane during an IS attack last week, and over 900 members of the Muslim Shaitat tribe who were killed in 2014 after opposing the Jihadist group.
There are no reliable figures in Iraq, but the group is believed to have executed thousands there, including as many as 1,700 mostly recruits at the Speicher military base near Tikrit.
Thousands more have died battling IS in Syria and Iraq, including Syrian rebels and government forces, Kurdish fighters in both countries, and Iraqi government troops and militias.
But few of those forces have had much success against the group, with the Iraqi army in particular facing criticism for abandoning territory to IS during a push by the Jihadist group in mid-2014.
Iraqi government forces have “no clear command structure,” said Zaid al-Ali, author of “The Struggle For Iraq’s Future”.
“Clearly Baghdad should have enough forces at its disposal to control territory, but not all the anti-IS forces take their instructions from Baghdad,” he added.
“Some operate as they please, some take instructions from elsewhere.”
In Syria meanwhile, only Kurdish forces backed by the US-led coalition have been able to effectively tackle the group, with analysts saying opposition forces and the regime appear to lack the weapons or the resolve to fight the Jihadists.
Even the anti-IS coalition, which is carrying out air strikes in Syria and Iraq and training Iraqi troops on the ground, has had limited success.
It has helped ground forces push IS from Kobane and Tal Abyad in Syria, and Tikrit and Diyala province in Iraq.
But the group has continued to score shocking victories, including the recent capture of Syria’s ancient town of Palmyra and the taking of the Iraqi city of Ramadi in mid-May.
“The international mobilisation against Daesh has been minimal,” said Sayegh, using the Arabic acronym for the group.
“But it may be that they cannot do more, because the return of 150,000 US troops to the battlefield is out of the question.”
Ultimately analysts say IS’s success is as much the result of political problems as it is military shortcomings.
IS has emerged because of “the failure of Syria and Iraq and the sectarian divisions in them, as well as corruption and decades of authoritarian rule,” Sayegh said.