Under the hard-won deal announced at a meeting of Arab foreign ministers in Cairo after months of prevarication by Damascus, the regime agreed to a complete halt to violence against civilians and to consultations by Arab mediators aimed at opening up a dialogue with the opposition.
"The Syrian delegation accepted the Arab League plan without reservations and in its entirety," the League official said.
The blueprint, a copy of which was obtained by AFP, provides for a "complete halt to the violence to protect civilians."
But on a day on which activists said that more than 30 more people were killed in violence, the White House said the only way forward was for Assad to quit.
"Our position remains that President Assad has lost his legitimacy to rule and should step down," White House spokesman Jay Carney said.
"We support all international efforts that are aimed towards convincing the regime to stop attacking its own people and perpetrating violence against its own people. But our position on Assad has not changed."
More than 3,000 people have died in the government's bloody crackdown on the unprecedented protests against Assad's rule which broke out in mid-March, according to UN figures.
It also calls for the "release of people detained as a result of the recent events, the withdrawal of forces from towns and districts where there have been armed clashes, and the granting of access to the Arab League, and Arab and international media."
It stipulates that "the Arab ministerial committee (headed by the prime minister of Qatar) will conduct consultations with the government and the various Syrian opposition parties aimed at launching a national dialogue."
The text does not specify a venue for the dialogue, a bone of contention between the government, which insists on Damascus, and the opposition which says it should be outside Syria.
UN chief Ban Ki-moon said it was vital that Assad's regime now swiftly implement the agreement in full.
"He must implement the agreement as soon as possible as agreed," Ban told a news conference in Tripoli.
"People have suffered too much for too long and it's an unacceptable situation," the UN chief said. "Killing civilians must stop immediately in Syria."
Qatari Prime Minister Sheikh Hamad bin Jassem al-Thani said "if Syria does not respect its commitments, the ministerial committee will meet again and take the necessary decisions."
Arab League chief Nabil al-Arabi said the main aim was "to provide an Arab solution which sends a clear message to the Syrian people of qualitative progress towards halting all forms of violence."
German Chancellor Angela Merkel, meanwhile, called for "stronger condemnation by the United Nations" of the killing of civilians in Syria, after a draft resolution put to the Security Council early last month was vetoed by China and Russia.
Ahead of the Cairo meeting, the opposition Syrian National Council accused the regime of escalating its crackdown on pro-democracy protesters in recent days, causing the deaths of dozens of civilians.
Such action, it said, was "Syria's response to the Arab League's plan."
More than 30 people were killed in violence on Wednesday, almost half of them security force personnel killed in clashes with troops who had mutinied rather than follow orders to shoot on civilians, a human rights group said.
Deserters killed 15 members of the security forces in two bombings of road convoys in the flashpoint central province of Hama, the Syrian Observatory for Human Rights said.
The Observatory said the attacks were carried out "in response to the massacre of 11 workers" earlier in the day by a pro-regime group in Homs, another province in central Syria which has been a centre of dissent.
The gunmen stormed a factory in the restive province, killing the 11, while security forces shot dead eight civilians in several Homs neighbourhoods, the rights watchdog said.
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