In a letter shared by Al Jazeera Arabic, Sinwar reassured the Palestinian group's leadership abroad about the armed wing's achievement after two-and-a-half months of Israeli bombing and ground operations.
He claimed that 5,000 Israeli soldiers and officers have been killed and wounded since the ground operations began in late October. One-third of them, around 1,660, were killed, he said, while the rest have been permanently disabled or seriously wounded.
The Israeli military says 156 soldiers have been killed in ground combat so far, and 600 more have been wounded.
Israeli media outlets have reported a significantly higher number of wounded soldiers, citing discrepancies between figures provided by the army and cases documented by hospitals.
Sinwar added that Palestinian fighters, who are using guerrilla warfare tactics, such as snipers, anti-tank missiles and explosive devices, have completely or partially damaged at least 750 Israeli armoured vehicles, including tanks.
"Izz al-Din al-Qassam Brigades are fighting a fierce and unprecedented battle against the Israeli occupation forces," Al Jazeera reported Sinwar as saying.
He added that the brigades are smashing the Israeli army and will continue to do so, and that they will not submit to the conditions of the occupation.
The Qassam Brigades are the largest Palestinian faction fighting the Israelis in Gaza. Others include Saraya al-Quds (the armed wing of Islamic Jihad) and the smaller Abu Ali Mustafa Brigades, which is aligned with the leftist Popular Front for the Liberation of Palestine.
Israel has reportedly proposed another temporary pause in fighting during which a group of Israelis could be released from Gaza in exchange for the release of some Palestinian prisoners.
Hamas has publicly rejected the offer, saying no prisoner exchanges will take place before an agreement is reached to end the war permanently and allow humanitarian aid into Gaza.
While the unprecedented scale of Israeli bombing has killed more than 20,000 people and pushed Gaza to the brink of a humanitarian disaster, Hamas fighters still appear able to inflict heavy losses on the Israeli military.
A total of 14 soldiers were killed over the weekend across the Gaza Strip, including in areas the army claims to have control over.
Since October 07, the Israeli army has released the names of 489 soldiers who have been killed in combat or died during the course of operations. That number includes former captives who likely died in Israeli bombardment.
Hamas and other Palestinian factions regularly publish videos of their attacks on Israeli positions, including images of Israeli weapons and ammunition seized during the fighting.
The Israeli military claims to have killed thousands of Hamas fighters and destroyed many tunnel shafts. However, no serious damage to the group's fighting capabilities has been acknowledged by the group so far.
