Violence has escalated in the Gaza Strip as Hamas fighters fired mortars and rockets at the offices of President Mahmoud Abbas.Over a dozen people have been killed since clashes erupted between rival factions of Mr Abbas's Fatah movement and opposition Hamas members, shattering a three-day-old ceasefire.Earlier, three Hamas gunmen and two members of Mr Abbas's intelligence service were killed while 25 people were wounded when the Hamas-led interior ministry was surrounded.The fragile truce broke down on Thursday after Hamas gunmen ambushed what Fatah said was a convoy carrying military equipment to Mr Abbas's presidential guard in Gaza. Six people were killed.Hundreds of Fatah gunmen then raided a Hamas-affiliated university in Gaza and seven Iranian citizens were arrested while an eighth reportedly committed suicide.Elsewhere, the Israeli army killed four Palestinian gunmen in two separate operations.Rival gunmen later fought street battles using mortar shells, rocket-propelled grenades and machine guns. At least 120 people were wounded.And the Fatah-affiliated Voice of Labour radio station in the northern town of Jebaliya was also blown up by Hamas gunmen after a five-hour siege.Since Hamas won parliamentary elections last year, over 100 Palestinians have been killed. At the time, President Abbas and his faction took control of the security forces and Hamas established its own militia.It has resulted in continuous fighting despite a series of brief truces.© Independent Television News Limited 2007. All rights reserved.