The official, who spoke on condition of anonymity because the group had not officially made the change public, said that Mr. Sinwar would appear publicly in the next few days to cement his political ascendance.
Mr. Sinwar’s rise indicates the growing power of Hamas’s hawkish military wing, according to analysts. Mr. Sinwar is slated to replace Ismail Haniya, a leader associated with the political wing of the movement that, at least nominally, deals more with diplomacy and governance and less directly with armed struggle.
Israel, the United States and Europe have designated all of Hamas as a terrorist organization and have rebuffed efforts by its political wing to gain international recognition. Mr. Haniya is now contending for the top post in Hamas’s political bureau abroad in place of Khaled Mashal.
Although Israeli officials have long rejected any clear distinction between the military and political wings of Hamas, arguing that they are one movement and have the same goals, Israeli experts described Mr. Sinwar as an extremist even by Hamas standards.
“First of all, you have to remember Sinwar is a terrorist,” said Kobi Michael, a former head of the Palestinian desk at Israel’s Ministry for Strategic Affairs and now a research fellow at the Institute for National Security Studies at Tel Aviv University.
“He represents the most radical and extreme line of Hamas,” Mr. Michael said. He added that Mr. Sinwar is affiliated with, and favors support from, Iran, as opposed to political wing leaders who were working for closer relations with neighboring Egypt, understanding that Egyptian assistance is necessary for improving living conditions and the reconstruction of war damage in the Gaza Strip.
Describing Mr. Sinwar as a “bitter enemy” of the Egyptians, Mr. Michael said that Mr. Sinwar also favored cooperation with Islamic State affiliates fighting the Egyptian Army in the Sinai Desert and that his election could destabilize the region.
Mr. Sinwar, who grew up in Khan Yunis, in the southern Gaza Strip, started out as one of a hard core of Muslim Brotherhood activists and part of the generation that formed Hamas. Even before the movement was formally established in late 1987, with the outbreak of the first Palestinian intifada, or uprising, against Israel, Mr. Sinwar helped set up an early prototype of Hamas’s military wing known as the Izz al-Din al-Qassam Brigades. It was called Al Majd, Arabic for glory.
Going on to found Hamas’s security branch, his job included punishing “morality” offenders and killing Palestinians suspected of collaborating with Israel. By early 1988 he was under arrest and was sentenced to four life terms in Israeli prison for attempted murder and causing grievous bodily harm through sabotage.
Israeli security experts said Mr. Sinwar was also involved in attacks against Israeli soldiers.
“He had the status of Prisoner No. 1,” Yaron Blum, a former senior official of Israel’s Shin Bet security agency, told Israel Radio. “He came by this honestly.”
Mr. Sinwar was released in 2011, one of a total of 1,000 Palestinian prisoners exchanged for a single Israeli soldier, Gilad Shalit, who was captured during a 2006 cross-border raid and was held by Hamas in Gaza for five years. The most senior prisoner released, and clearly destined for leadership, Mr. Sinwar said that his time in jail was spent studying his enemy and, in the latter stages, taking part in the negotiations for the prisoner swap.
“They wanted the prison to be a grave for us. A mill to grind our will, determination and bodies,” Mr. Sinwar said at the time. “But thank God, with our belief in our cause we turned the prison into sanctuaries of worship and to academies for study.”
Mr. Sinwar has since consolidated his power and his harsh reputation. Mr. Sinwar is believed to have been behind the detention, torture and killing of a Hamas commander, Mahmoud Ishtiwi, who was initially held for embezzlement in 2015. Months later he was killed, accused of committing “moral crimes” – amid suspicions of gay sex — that Mr. Sinwar apparently feared could lead to extortion and compromise Hamas.
Ibrahim Madhoun, a Gaza-based writer on Hamas, said that Mr. Sinwar’s assumption of power was not quite a takeover by the military wing. His previous position was akin to a defense minister rather than a chief of staff, Mr. Madhoun said by telephone, adding, “He is more than a military man, he is a leader.”
Mr. Sinwar is also respected in Gaza for his ability to balance between political and military circles. Al Resalah, a Hamas-affiliated newspaper in Gaza, credited Mr. Sinwar with ensuring coordination between Hamas’s Qassam Brigades and the leadership during wartime, when senior commanders were in hiding to avoid Israeli assassination.
Israel has fought three wars against Hamas and the other armed groups in Gaza since 2008 and has been constantly warning that Hamas has been building up its rocket and underground tunnel network since the last cease-fire came into effect in the summer of 2014, in preparation for the next round of fighting.