Sinwar, who last month replaced slain Hamas leader Ismail Haniyeh, said in a letter to the group's Yemeni allies that "we have prepared ourselves to fight a long battle of attrition".
Deadly fighting meanwhile raged on in the Gaza Strip, where medics and rescuers said Israeli strikes on Monday -- which the military has not commented on -- killed at least two dozen people.
The latest strikes came as Israeli Defence Minister Yoav Gallant warned that prospects for a halt in fighting with Hezbollah militants in Lebanon were dimming, yet again raising fears of a wider regional conflagration.
Senior Hamas official Osama Hamdan told AFP at the weekend the group "has a high ability to continue" fighting despite losses, noting "the recruitment of new generations" to replace killed militants.
Gallant last week said Hamas, whose October 7 attack triggered the war, "no longer exists" as a military formation in Gaza.
Sinwar, in his letter to Yemen's Huthis, threatened that Iran-aligned groups in Gaza but also elsewhere in the region including Lebanon and Iraq would "break the will of Israel" after more than 11 months of war.
Independent UN rights experts warned that Israel risked becoming an international "pariah" over its actions in Gaza and called on Western countries to ensure accountability.
The October 7 attack on southern Israel that sparked the war resulted in the deaths of 1,205 people, mostly civilians, according to an AFP tally based on official Israeli figures.
Militants also seized 251 hostages, 97 of whom are still held in Gaza, including 33 the Israeli military says are dead.
Israel's retaliatory military offensive has killed at least 41,226 people in Gaza, according to the Hamas-run territory's health ministry, which does not provide a breakdown of civilian and militant deaths.
- Israel-Hezbollah tensions surge -
Tensions have surged along Israel's northern border with Lebanon, amid fears the violence could explode into an all-our war.
"The possibility for an agreement is running out as Hezbollah continues to tie itself to Hamas and refuses to end the conflict," Gallant told visiting US envoy Amos Hochstein, a defence ministry statement said.
Israeli media outlets said Prime Minister Benjamin Netanyahu was considering firing Gallant, but the premier's office denied the reports.
Gallant, who had already survived an attempt by Netanyahu to dismiss him in March 2023, is among several Israeli officials who have been at odds with the Israeli leader on war policy.
Netanyahu told Hochstein later Monday he seeks a "fundamental change" in the security situation on Israel's northern border.
Lebanon's Iran-backed Hezbollah group has traded near-daily cross-border fire with Israeli forces since October 7 in stated support of ally Hamas.
Hezbollah deputy chief Naim Qassem said Saturday his group has "no intention of going to war", but if Israel does "unleash" one "there will be large losses on both sides".
The cross-border violence since early October has killed 624 people in Lebanon, mostly fighters but also including at least 141 civilians, according to an AFP tally.
On the Israeli side, including in the annexed Golan Heights, authorities have announced the deaths of at least 24 soldiers and 26 civilians.
- Deadly strikes -
In central Gaza, survivors scoured debris Monday after a strike on the Nuseirat refugee camp.
Ten people were killed and 15 others were wounded when an air strike hit the Al-Qassas family home in Nuseirat in the morning, said a medic at Al-Awda Hospital, where the bodies were taken.
"My house was hit while we were sleeping without any prior warning," said survivor Rashed al-Qassas.
Gaza's civil defence said six Palestinians were killed in a similar strike at night on a house belonging to the Bassal family in Gaza City's Zeitun neighbourhood.
Emergency services later reported six more deaths, with Al-Awda Hospital saying it received the bodies of three people killed in Israeli strikes on Nuseirat.
The Gaza war has drawn in Iran-backed Hamas allies across the Middle East, including Hezbollah in Lebanon and Yemen's Huthis, who on Sunday claimed a rare missile attack on central Israel which caused no casualties.
Netanyahu in response said: "They should have known by now that we charge a heavy price for any attempt to harm us."
In July, a Huthi drone strike killed a civilian in Tel Aviv, at least 1,800 kilometres from Yemen, prompting retaliatory strikes that caused significant damage and deaths at Yemen's rebel-controlled Hodeida port.
Since November the Huthis have targeted Israel and its perceived interests in stated solidarity with Palestinians in Gaza, launching strikes that have disrupted global shipping through vital waterways off Yemen.
In a televised speech, the Huthis' leader said the rebels and their regional allies were "preparing to do even more".
"Our operations will continue as long as the aggression and siege on Gaza continue," Abdul Malik al-Huthi said.
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