UN Secretary-General Antonio Guterres said Thursday that there was a "flame of hope" in Syria following the fall of president Bashar al-Assad's regime but warned of significant challenges ahead.

He also underlined that Israel's widespread strikes on Syrian military infrastructure were "violations" of the country's sovereignty and called for them to cease.

"The Middle East is being consumed by many fires, but today, there is a flame of hope in Syria, and that flame must not be extinguished," he said.

As Syrians pour out of prisons that Bashar al-Assad used to cement his grip on power, the Saudi-based family of one detainee is desperate for any news, even if it confirms their worst fears.

Rania al-Abassi, a Syrian dentist and former national chess champion, disappeared into Assad's lethal network of detention facilities in 2013 along with her husband and their six children.

In the villages above the Syrian port city of Tartus they once hailed the sons who died fighting in Bashar al-Assad's service as martyrs.

But mothers are now nervous to display their sons' pictures, and no longer hide their bitterness towards the ousted leader for whom they sacrificed so much.

"It's true my son is dead," said Jamila Jabr, the 60-year-old mother of Humam, an army conscript who was killed in combat in 2012.

"But the important thing is that Bashar al-Assad is gone. He destroyed us and destroyed our children's futures and starved us."

Israel on Thursday faced accusations of "acts of genocide" and exhibiting "signs of ethnic cleansing" in its military offensive in Gaza, charges Israeli authorities dismissed as "lies".

They are the latest in a series of accusations levelled against Israel -- and denied by the country -- during its 14-month war against Palestinian Hamas militants in the Gaza Strip.

Georgina Deiratani, a Syrian Christian who has lived in Moscow for over two decades, says she is still in shock at the ouster of president Bashar al-Assad and his escape to Russia.

"Everybody is saying 'congratulations'," said the 37-year-old, one of several thousand Syrians estimated to be living in the Russian capital.

"But for me this is the end of Syria."

She remembers growing up in a country where different communities lived in harmony, but makes no mention of the brutality of the half-century dictatorship of Assad and his father before him.

Yemen's Iran-backed Huthi rebels said Israeli air strikes on Thursday killed nine people, after the group fired a missile toward Israel, badly damaging a school.

Israeli Prime Minister Benjamin Netanyahu threatened further retaliation.

The Huthis, who control most of Yemen's population centres, have repeatedly launched missiles at Israel since the Gaza war broke out more than a year ago.

Israel has previously hit back against targets in Yemen, but Thursday's were the first against the rebel-held capital Sanaa.

US Secretary of State Antony Blinken called Wednesday on Syria's triumphant HTS rebels to follow through on promises of inclusion, saying it can learn a lesson from the isolation of Afghanistan's Taliban.

The Islamist movement rooted in Al-Qaeda and supported by Turkey has promised to protect minorities since its lightning offensive toppled strongman Bashar al-Assad this month following years of stalemate.

US Secretary of State Antony Blinken said Wednesday he remained hopeful of reaching a ceasefire between Israel and Hamas in Gaza and promised to use his remaining month in office to achieve it.

Blinken, however, declined to predict success after repeated disappointments in his government's efforts to end 14 months of brutal war in the Palestinian territory.

"Look, I'm hopeful. You have to be. We're going to use every minute of every day of every week that we have left to try to get this done," said Blinken, who leaves office on January 20.

US Secretary of State Antony Blinken said Wednesday that President-elect Donald Trump had a window to negotiate with Iran, saying the development of a nuclear weapon by Tehran was not inevitable.

The outgoing top US diplomat acknowledged that the cleric-run state may be more seriously considering a nuclear weapon after military setbacks in the region.

In the last year, Israel's military has hit Iranian air defenses and weakened its Lebanese ally Hezbollah, while rebels have toppled its main Arab ally in Syria, Bashar al-Assad.

Turkey on Wednesday rejected US President-elect Donald Trump's claim that the rebel ouster of Syrian strongman Bashar al-Assad was an "unfriendly takeover" by Ankara.

"We wouldn't call it a takeover, because it would be a grave mistake to present what's been happening in Syria" in those terms, Foreign Minister Hakan Fidan told broadcaster Al Jazeera in an interview.

"For Syrian people, it is not a takeover. I think if there is any takeover, it's the will of the Syrian people which is taking over now."