Israel on Tuesday retaliated with airstrikes against a rocket fired from the Gaza strip, hours after it said it had discovered a tunnel leading from Palestinian into Israeli territory, the army said. "IDF (Israel Defence Forces) fighter jets and attack helicopter struck an underground infrastructure belonging to the Hamas terror organisation in the southern Gaza Strip" following the rocket attack, the military said in a statement. Palestinian security sources said the aircraft hit two targets in Khan Yunis and Deir Al Balah belonging to Hamas, the Islamist movement hostile to Israel that has held power in Gaza since 2007. The IDF had earlier said that the rocket fired from the Khan Yunis area "was intercepted by the Iron Dome" aerial defence system, without indicating if it had caused any casualties or damage. Israel discovered the new cross-border tunnel from the Gaza Strip on Tuesday that its military said extended “dozens of metres underground” and into Israeli territory. The military said its engineers found the tunnel using underground sensors attached to a concrete barrier that, once completed, will run 65 kilometres around Gaza. It said Palestinians have used underground tunnels to smuggle in commercial goods to Gaza, and bring in weapons for militants from the Strip’s ruling Hamas group and other factions. Army spokesman Jonathan Conricus said the tunnel was a violation of Israel's sovereignty. He said Israel did not know who had dug the tunnel, but said it held Hamas responsible for all activity in the Palestinian enclave. Militants have also used the tunnels to launch attacks inside Israel, which maintains a land and sea blockade of Gaza. Israel has said that the reason it blockades the strip of land is because of the threat from Hamas. The entrance of the tunnel lies in the city of Khan Younis in southern Gaza, extending across the Israeli border before ending underground before the barrier, military spokesman Jonathan Conricus said. “We have not seen an exit point from the tunnel," he said. "So you could deduce from that that the aim was not for the terrorists to emerge from that location, but rather further inside Israel." He said the military had not yet determined who had built the tunnel, but he assumed it was Hamas. Israel and Hamas last fought a large-scale war in 2014 and have engaged in many smaller cross-border confrontations since. Hamas and other Gaza militants have defended the tunnels as part of what they call their preparation for fighting. Mr Conricus said the military would “neutralise” the tunnel in the coming days. The military has discovered around 20 tunnels since the 2014 war, he said.