FACTBOX: Hamas leaders struggle for survival and unity

(Reuters) - Israel's three-week-old assault on the Gaza Strip has magnified a challenge long familiar to Hamas leaders: how to evade death at the hands of the Israeli military and maintain political cohesion at the same time.

Saeed Seyyam, the Hamas security chief in Gaza, was killed in an Israeli air strike on January 15.

The Palestinian Islamist group makes decisions by consensus -- no easy task when its chief is in exile and his politburo has some members in Gaza, some in the occupied West Bank and others in Israeli jails. They must also coordinate with the powerful underground military leadership of Hamas's armed wing.

Here are some of Hamas's top known figures. The group has kept the identity of its Gaza leader a secret since Israel killed his predecessor, Abdul-Aziz al-Rantissi, in 2004.

* Khaled Meshaal, based in Damascus, is believed to have the last say in political and military decision-making. He has taken a defiant line in the Gaza conflict, saying Hamas would not consider a truce before the Jewish state ended its offensive and lifted an almost two-year-old blockade of the territory.

* Mohammed Deif, the group's clandestine military chief, has long topped Israel's wanted list for masterminding attacks, including suicide bombings, against Israelis since the 1990s.

* Ahmed al-Jaabari is deputy commander of the Izz el-Deen al-Qassam military wing, believed to have 25,000 fighters.

* Ismail Haniyeh, pragmatic, popular and outspoken, has run the Hamas government in Gaza since President Mahmoud Abbas fired him as Palestinian prime minister in 2007. Although respected inside Hamas, he may not have much clout over the military wing.

* Mahmoud al-Zahar is a political leader who may have more influence over the armed wing because of his hardline views. He has lost two sons in the conflict against Israel.

* Khalil al-Hayya heads the majority Hamas parliamentary bloc. He acted as senior negotiator in 2007 during the conflict between Hamas and forces loyal to Abbas, which ended in a Hamas seizure of the Gaza Strip in June that year.

* Saeed Seyam, in charge of the interior ministry in Gaza, was killed in an Israeli air strike on January 15. Seyyam founded Hamas's Executive Force and, after its incorporation into a security service for Gaza, supervised 13,000 Hamas police and security men, many of whom are involved in fighting Israel.

(Jerusalem newsroom; Editing by Diana Abdallah)