Sinwar is among the Hamas leaders accused of masterminding the group's attack on Israel on October 7 last year, and is said to have been hiding in a network of tunnels the militants built under Gaza.
On Thursday, Israel said it was checking whether it had killed 61-year-old Sinwar in a Gaza operation, in what would constitute a massive blow to the group.
Sinwar was the head of Hamas in Gaza during the October 7 attack, but rose to become the group's overall leader in August after the killing of political chief Ismail Haniyeh.
The killing of Haniyeh in Tehran on July 31 has been widely blamed on Israel, which has never claimed the assassination.
Also in July, Israel said it killed Hamas military leader Mohammed Deif, but the group never confirmed his death.
"Every Hamas member is a dead man," Israeli Prime Minister Benjamin Netanyahu said last year after the October 7 attack sparked the Gaza war.
Punishing collaborators
The Hamas attack resulted in the deaths of 1,206 people in Israel, according to an AFP tally based on Israeli official figures that include hostages killed in captivity.
Israel's retaliatory campaign in Gaza has killed 42,438 people, the majority civilians, according to data from the health ministry in the Hamas-run territory.
The October 7 attack probably a year or two in the planning, "took everyone by surprise" and "changed the balance of power on the ground", said Leila Seurat of the Arab Centre for Research and Political Studies in Paris.
Sinwar is a security operator "par excellence", according to Abu Abdallah, a Hamas member who spent years alongside him in Israeli jails.
"He makes decisions in the utmost calm, but is intractable when it comes to defending the interests of Hamas," Abu Abdallah told AFP in 2017, after his former co-detainee was elected Hamas's leader in Gaza.
Born in the Khan Yunis refugee camp in southern Gaza, Sinwar joined Hamas when Sheikh Ahmad Yassin founded the group around the time the first Palestinian intifada began in 1987.
Sinwar set up the group's internal security apparatus the following year and went on to head an intelligence unit dedicated to flushing out and mercilessly punishing -- sometimes killing -- Palestinians accused of providing information to Israel.
According to a transcript of an interrogation with security officials published in Israeli media, Sinwar professed to have strangled an alleged collaborator with a keffiyeh scarf in a Khan Yunis cemetery.
A graduate of the Islamic University in Gaza, he learned perfect Hebrew during his 23 years in Israeli jails and is said to have a deep understanding of Israeli culture and society.
'Radical and pragmatic'
He was serving four life terms for the killing of two Israeli soldiers when he became the most senior of 1,027 Palestinians released in exchange for Israeli soldier Gilad Shalit in 2011.
Sinwar later became a senior commander in the Ezzedine al-Qassam Brigades, the military wing of Hamas, before taking overall leadership of the movement in Gaza.
While his predecessor, Haniyeh, had encouraged efforts by Hamas to present a moderate face to the world, Sinwar has preferred to force the Palestinian issue to the fore by more violent means.
Sinwar is said to have strived for a single Palestinian state bringing together the Gaza Strip, the occupied West Bank -- controlled by Mahmud Abbas's Fatah party -- and annexed east Jerusalem.
According to US think-tank the Council on Foreign Relations, he has vowed to punish anyone obstructing reconciliation with Fatah, the rival political movement with which Hamas engaged in factional fighting after elections in 2006.
That coming together remains elusive, but the prisoner releases resulting from the brief November truce agreement with Israel saw Hamas's popularity soar in the West Bank.
Sinwar has pursued a path of being "radical in military planning and pragmatic in politics", according to Seurat.
"He doesn't advocate force for force's sake, but to bring about negotiations" with Israel, she said.
The Hamas chief was added to the US list of the most wanted "international terrorists" in 2015.
Vowing in November to "find and eliminate" Sinwar, Israeli Defence Minister Yoav Gallant urged Gazans to turn Sinwar in, adding "if you reach him before us, it will shorten the war".
