Since 2005's disengagement from the Gaza Strip, Hamas has forged a formidable military of 20,000 men, many of whom have been trained in Iran and Lebanon, an Israeli think tank said in report issued on Thursday. Entitled "Hamas's Military Buildup in the Gaza Strip," the report - compiled by the Intelligence and Terrorism Information Center - detailed the structure of the Hamas military force in Gaza, naming commanders of its various brigades and the types of weapons it had succeeded in smuggling in from Egypt via tunnels underneath the Philadelphi Corridor. According to the think tank, which receives its information from the Shin Bet (Israel Security Agency), Hamas's military buildup will take a few years to finish. Hamas's military capabilities are based on the Izzadin Kassam Brigades - the group's military wing, responsible for terror attacks against Israel. In the event of a large-scale Israeli invasion of Gaza, the report said that Hamas would refrain from fighting against the IDF in open areas but would instead channel the fighting into more densely populated areas. "Hizbullah's success in providing an asymmetric response to the IDF's might during the Second Lebanon War made it a role model for Hamas," the report reads. "The lessons of the Second Lebanon War also illustrated the importance of having a strong military force which could stand fast and survive to protect Hamas's control of the Gaza Strip, like the military force Hizbullah established in south Lebanon." The commander of the Kassam Brigades, made of 10,000 men, is Ahmed Ja'abri, who took over following an assassination attempt on Muhammad Deif in July 2006 that left the supreme Hamas commander seriously disabled. It is unclear how much influence Deif has over the force now. Hamas also controls the Executive Force, which numbers another 10,000 men. There are another 3,000 to 4,000 gunmen who belong to other terror groups - such as Islamic Jihad and the Popular Front for the Liberation of Palestine - and these would likely join Hamas's fight against the IDF in the event of an Israeli invasion, the report said. According to the report, the northern Gaza brigade under the Kassam Brigade umbrella is commanded by Ahmed Ghandour; two brigades in Gaza City are commanded by Ja'abri; the central Gaza brigade is commanded by Ayman Nawfal, who is currently in prison in Egypt; two brigades in southern Gaza - one in Khan Yunis and one in Rafah - are commanded by Muhammad Sinwar and Ra'ed al-Atar, respectively. Hamas also has a coastal force of around 200 men and several vessels, again according to the report. Hamas gets it arms from three sources - Iran and Syria (sometimes directly and sometimes via Hizbullah), arms dealers and independent production, according to the researchers. It has several dozen long-range 122 mm. Grad-model Katyusha rockets - the type fired into Ashkelon in February; dozens of anti-tank missiles - including advanced Sagger missiles and thousands of rocket-propelled grenades; several anti-aircraft missiles; and a few dozen anti-aircraft machine guns. Hamas also has various types of listening equipment for intelligence gathering, and an unknown quantity of night-vision equipment. Hamas's rocket arsenal is based on several hundred independently produced Kassam rockets, with diameters ranging from 90 mm. to 115 mm. and ranges of nine-13 kilometers. Hamas has also received an unknown number of Iranian-made long-range mortar shells. The Hamas military wing has several independently produced longer-range rockets which can reach 19 km., and dozens of standard long-range Grad rockets (122 mm.), with a range of up to 20.4 km. that were smuggled into Gaza or confiscated from the Palestinian Authority security services following the Hamas takeover of the Gaza Strip in June 2007. The breach of Rafah crossing to Sinai in January allowed Hamas to acquire additional standard rockets, and perhaps even a number of rockets with ranges longer than 20.4 km. that can reaching targets north of Ashkelon. The report warned that the current period of relative calm with Hamas was allowing the group to rehabilitate its military infrastructure damaged during the IDF's Operation Hot Winter in early March. "The calm that has settled on the Gaza Strip will allow Hamas to continue its military buildup while repairing the damage and overcoming the weak spots which became apparent during the latest round of escalation," the report said.