FACTBOX: Facts about Palestinian group Hamas

(Reuters) - An Israeli air and ground offensive against Hamas militants in the Gaza Strip has devastated the territory for three weeks, with the aim of dealing a crippling blow to the Islamist group and ending cross-border rocket fire.

Hamas, which rules the coastal enclave, has continued to fire rockets into southern Israel, but Israeli forces pushed into the Gaza Strip's major cities with relative ease.

Here are some facts about Hamas:

CAPABILITIES

* Before Israel's offensive began on December 27, Hamas was believed to have thousands of short-range rockets, as well as hundreds of longer-range Grad rockets that could strike Israeli population centers up to 40 km (25 miles) from the border with the Gaza Strip. Israel says it has destroyed much of the group's arsenal, but Hamas has fired daily salvoes farther than ever before, striking major Israeli cities.

* Despite suffering substantial casualties, Hamas has killed only five Israeli soldiers in the invasion force. Four Israelis were killed by "friendly fire" and one was hit by a rocket that landed in Israel early in the war. Israeli soldiers had expected a tougher fight and were surprised how few casualties they took.

* Hamas has little defense against the Israeli air force, the most technologically advanced in the region. The Israeli military has said its ground vehicles have been hit by anti-tank weapons and land mines, but that none of its aircraft has been damaged by Hamas fire. There is little evidence that Hamas has used shoulder-fired anti-aircraft missiles, a weapon which the Israelis had suspected the Islamist fighters possessed.

* First deployed on the streets of Gaza in May 2006, Hamas's main security apparatus, initially dubbed the Executive Force, grew from an estimated 3,000 members to more than 13,000, divided into several security branches. Hamas has not said how many of its gunmen have been killed in the fighting, but Israel says it has killed hundreds of Hamas militants. The Executive Force's founder Saeed Seyyam was killed by Israel on January 15.

* Hamas's main security force has grown since its takeover of the Gaza Strip in June 2007, in part because the Islamist group has money to pay salaries. The core of the force is mostly members of the Hamas armed wing, the Izz el-Deen al-Qassam Brigades, but it includes some members from allied militant factions such as the Popular Resistance Committees.

* The Qassam Brigades were believed to have 25,000 members but it is unclear how many of them overlap with other units.

* Hamas's forces have assault rifles and rocket-propelled grenades smuggled into Gaza through tunnels from Egypt.

* Israel and the United States say Iran provides funding and training to Hamas units. Hamas has used fortified positions, modeled on those used by Hezbollah guerrillas in southern Lebanon, to attack advancing Israeli troops.

* Hamas has called for a new uprising against Israel that would include suicide bombings in Israeli cities. But Gaza is cordoned off and many Hamas supporters in the West Bank have been rounded up by Israel and Palestinian security forces loyal to President Mahmoud Abbas. Israeli security officials say the barrier the Jewish state is building in and around the occupied West Bank has made it harder to launch suicide operations.

* It is unclear how far Hamas's capabilities and its support among the general population have been affected by the war.

TIMELINE

1987 - Hamas established during first Palestinian uprising.

1993 - Hamas rejects interim peace accords between Israel and Yasser Arafat's Palestine Liberation Organization

1996 - Arafat elected Palestinian president. Fatah wins 68 out of 88 seats in parliament. Hamas boycotts the vote.

2000 - Second Intifada, or uprising, starts after the collapse of peace talks.

2004 - Arafat dies in Paris and is succeeded by Mahmoud Abbas as head of the PLO and Palestinian president.

2005 - Abbas and Israeli leader Ariel Sharon declare ceasefire. Abbas wins support from Hamas, which largely abides by the truce. Israel ends 38-year Gaza occupation.

2006 - Hamas wins Palestinian parliamentary election, taking 74 seats in 132-member parliament. Abbas's once-dominant Fatah movement wins 45 seats.

2007 - Hamas Islamists win brief civil war in the Gaza Strip against more secular Fatah forces loyal to Abbas. Abbas sacks Hamas government and appoints his own in the Israeli-occupied West Bank.

WHAT IS HAMAS?

* Hamas is an acronym for Islamic Resistance Movement and also means "zeal" in Arabic.

* It is formally committed to destroying Israel and putting an Islamic state in its place. Hamas has said it would accept a Palestinian state in lands captured by Israeli forces in the 1967 war in return for a long-term truce.

* Hamas has rebuffed demands from Israel, the United States and other nations to recognize the Jewish state and disarm. Israel and the United States have sought to isolate Hamas.

* Hamas spearheaded a suicide bombing campaign against Israel starting in 2000 but later abided by a 2005 truce.

* Hamas was helped in the 2006 parliamentary election by its pledges to eradicate graft in the Palestinian Authority and its fight against Israel. It is also popular because of a network of charities it runs.

(Jerusalem Newsroom)