Hamas chief Yahya Sinwar, the 'mastermind' of Oct 7 attack who was Israel's most wanted

After a career in the shadows, spent in Israeli prisons and the internal security apparatus of Hamas, Yahya Sinwar emerged as the leader of the Palestinian group after helping ignite a full-blown war that has engulfed the region.

Sinwar was among the Hamas leaders accused of masterminding the group's attack on Israel on October 7 last year, and was said to have been hiding in a network of tunnels the militants built under Gaza.

The Israeli military on Thursday announced the killing of Sinwar by its forces in southern Gaza the previous day.

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.

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 was 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 was said to have a deep understanding of Israeli culture and society.

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".