Syrians are only now beginning to scratch the surface of the atrocities committed, after the former despot fled the country for Russia.
"We need to see of course justice and accountability for crimes," UN envoy Geir Pedersen said after arriving in Damascus.
"And we need to make sure that that goes through a credible justice system, and that we don't see any revenge."
Pedersen also called for "increased, immediate" aid to war-ravaged Syria, saying it had been through "an enormous... humanitarian crisis".
"We need to make sure that Syria receives increased, immediate humanitarian assistance," he said.
Assad fled Syria last Sunday following an 11-day rebel offensive led by the Islamist militant group Hayat Tahrir al-Sham (HTS), bringing to a dramatic end more than 50 years of Assad rule.
It came more than 13 years into the civil war sparked by Assad's violent crackdown on pro-democracy protests in 2011.
The war has killed upwards of 500,000 people and displaced more than half the country's population.
In the week since the rebels took Damascus, each day has seen more light shed on the depths of the despair visited upon Syria's people over the past decades.
Journalist Mohammed Darwish was one of those held in the so-called Palestine Branch, a jail run by Syria's feared intelligence services.
"I was one of those they interrogated the most," Darwish told AFP as he returned to the prison years after his ordeal. He said he was questioned "every day, morning and night" for 120 days.
Back to school
Calm is slowly returning to the streets of Damascus, with dozens of children streaming back to school Sunday for the first time since Assad fled.
An official at one school said "no more than 30 percent" were back on Sunday, but "these numbers will rise gradually".
On the diplomatic front, US State Secretary Antony Blinken said Saturday Washington had "been in contact with HTS and with other parties," without specifying how.
After meeting in Jordan, Western and Arab states along with Turkey -- a key backer of anti-Assad rebels -- called for a united peaceful Syria.
In a joint statement, diplomats from the United States, Turkey, the European Union and Arab countries called for a Syrian-led transition to "produce an inclusive, non-sectarian and representative government formed through a transparent process", with respect for human rights.
France said a four-strong diplomatic team would arrive on Tuesday to "retake possession of our real estate" as well as "establishing initial contact" with the new authorities and "evaluating the urgent needs of the population", acting Foreign Minister Jean-Noel Barrot said.
A Qatari delegation was due in Syria Sunday to meet transitional government officials for talks on aid and reopening its embassy.
Unlike other Arab states, Qatar never restored diplomatic ties with Assad after a rupture in 2011.
Turkey was ready to provide military support to Syria's new Islamist-led government set up by rebels if it requests it, Defence Minister Yasar Guler said on Sunday.
Guler said the new leadership should be given "a chance" and that Ankara was "ready to provide the necessary support" if needed, in remarks reported by Turkish media.
Sunni Muslim HTS is rooted in Syria's branch of Al-Qaeda and is designated a "terrorist" organisation by many Western governments.
Although it has sought to moderate its rhetoric in recent years, its seizure of power has sparked concerns both domestically and internationally over the protection of religious and ethnic minorities.
The interim government insists that the rights of all Syrians will be protected, as will the rule of law.
On Sunday, Syrian Christians attended their first church service since Assad's fall.
Pubs and stores selling alcohol in Damascus initially closed following the rebel victory, but are now tentatively reopening.
The landlord of one Damascus bar said the rebels told him: "'You have the right to work and live your life as you did before'."
Israeli strikes
Assad was propped up by Russia, Iran and Lebanon's Hezbollah militant group.
The rebel onslaught began on November 27, the day a ceasefire took effect in the Israel-Hezbollah war in Lebanon, in which Assad's ally suffered staggering losses.
Hezbollah leader Naim Qassem admitted Saturday that with Assad gone, his group could no longer be supplied militarily through Syria.
He also said he hoped Syria's new rulers saw Israel "as an enemy" and do not normalise ties with the country.
Both Israel and Turkey have carried out military strikes inside Syria since Assad's fall.
The Syrian Observatory for Human Rights on Sunday reported new Israeli strikes near Damascus, after 60 strikes across Syria on Saturday.
The Britain-based Observatory, which relies on a network of sources in Syria, reported attacks on Syrian army tunnels and arms depots in the Damir area.
Israel has also ordered troops into a UN-patrolled buffer zone separating Israeli and Syrian forces on the Golan Heights, a move the UN said violated a 1974 armistice.
But "the general exhaustion in Syria after years of war and conflict does not allow us to enter new conflicts", he said in an online statement.
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