Hamas has ruled Gaza with an iron hand since the 2007 takeover, deepening the political split with Abbas. Repeated attempts at reconciliation have failed.
The takeover deepened the isolation of Hamas and prompted the group to rely increasingly on Iran and Syria. Jabari was instrumental in developing the Hamas military arsenal and the group's networks in Iran, Sudan and Lebanon.
The Hamas founding charter calls for the destruction of Israel, though the group has been grappling with its political direction in recent years.
Founded in Gaza in 1987, Hamas carried out scores of suicide bombings in Israel, killing hundreds of Israelis, but halted most such attacks several years ago. Gaza militants, including those from Hamas, have fired thousands of mortars and rockets at Israel in the past decade, drawing Israeli retaliation.
Jabari was considered close to Hamas hard-liner Mahmoud Zahar, a Gaza strongman. In 2011, Jabari wrote in a Hamas publication that "as long as the Jews occupy our land, they have one thing (in store), death, or they leave the occupied Palestinian territories."