Gaza, June 18 : Israeli warplanes struck military posts of the al-Qassam Brigades, the armed wing of Islamic Hamas movement, in the Gaza Strip, Palestinian security sources said. Israeli media reported that the airstrikes on Thursday were in retaliation to the launching of dozens of incendiary balloons into southern Israel from Gaza for the third consecutive day, reports Xinhua news agency. The Palestinian sources and witnesses said that several explosions were heard in the northern and southern Gaza Strip following the airstrikes. The sources said that severe damage was caused to the military posts, but no injuries were reported. The Israeli media reports said that the incendiary balloons launched from Gaza caused at least eight fires in communities close to the borders. Launching incendiary balloons at Israel has been going on since Tuesday, in protest against the Israeli rightists' holding of a flag march in East Jerusalem. The Israeli Army said on Wednesday that it had carried out the first airstrike in the Gaza Strip since the ceasefire which came into effect on May 21 after the 11-day bloodshed in the besieged enclave. In the attack, Israeli war jets struck military compounds belonging to Hamas in the city of Khan Yunis in the southern Gaza Strip. The Army accused Hamas of being responsible "for all events transpiring in the Gaza Strip" and warned that the group "will bear the consequences for its actions". Hundreds of Israeli ultra-nationalists marched on Tuesday in East Jerusalem, waving Israeli flags. Some of them chanted "Death to Arabs" and other anti-Arab slurs. /IANS