Hamas security forces in the Gaza Strip on Saturday detained two members of an Islamist group suspected of firing rockets at Israel, a source close to the Palestinian group said. The source said the militants, who belonged to a Salafist organisation called Al-Tawfid wal Jihad, were detained in Gaza City by Hamas security forces who suspected them of being behind rocket attacks on the Jewish state. On Thursday night, a rocket was fired at the southern town of Kiryat Gat without causing any harm, the Israeli army said. In response, an Israeli warplane attacked a group suspected of firing the rocket, a military spokeswoman said. Israeli retaliation for attacks from Gaza has focused on targeting bases of the Ezzedine al-Qassam Brigades, the military branch of Hamas. The past month has seen an increase in rockets and other projectiles fired at Israel from Gaza after several months of calm following a flare-up in April when an anti-tank missile hit an Israeli school bus, killing a teenager. Israel responded to that attack with a series of air strikes that killed at least 19 Palestinians in the deadliest violence since Israel's devastating 22-day assault on Gaza in December 2008-January 2009. The violence raised fears of another offensive, but the Islamist movement Hamas which rules the Palestinian territory announced on April 10 a return to the truce that ended Israel's "Operation Cast Lead."