A resolution approved by Arab League ministers meeting in Cairo called for "opening communication channels with the Syrian opposition and providing all forms of political and material support to it". It also urged the Syrian opposition to unite.
The resolution solidifies Assad's ostracism among his Arab neighbours and will add to diplomatic pressure on Russia and China, which vetoed UN action on Syria on February 4, to lift their objections and allow the world body to act. It said violence against civilians in Syria had violated international law and that perpetrators deserved punishment. It scrapped an Arab League monitoring mission.
The resolution did not make clear whether the proposed joint UN-Arab peacekeeping force would involve armed troops, as in previous UN missions.
If so, it would be the second time in less than a year the Arab League had called for outside intervention in an Arab state. It's decision to back a no-fly zone in Libya last March led to Western bombing that helped bring down Muammar Gaddafi.
Syria's uprising, in which the United Nations says more than 5,000 people have died, has become one of the bloodiest of the Arab Spring revolts sweeping the region since the end of 2010. It has entered a more violent phase in recent weeks with a government crackdown on the restive city of Homs. Any peacekeeping mission would require a concensus from foreign powers, who have been divided on how to resolve a conflict that is descending into a civil war.
Moscow and Beijing drew strong criticism from the West after they prevented the Security Council on February 4 from backing an Arab plan that called for Assad to give up his powers.
Syrian television quoted its ambassador to the League as calling its decision "a flagrant departure from the group's charter and a hostile act that targets Syria's security and stability".
"This decision reflects the state of hysteria and blundering that the governments of some Arab countries are living in, particularly Qatar and Saudi Arabia after their latest failure at the UN Security Council to get foreign intervention in Syria."
Earlier on Sunday, Tunisia said it would host the first meeting on February 24 of a "Friends of Syria" contact group made up of Arab and other states and backed by the West. A similar Libya contact group played a vital role in co-ordinating Western and Arab aid to that country's rebels last year.
"The Syrian people deserve freedom as much as their brothers in Tunisia, Egypt, Libya and other Arab states that witnessed major political change," Tunisian Foreign Minister Rafik Ben Abdessalem told the ministers.
Saudi Foreign Minister Saud al-Faisal said: "How long will we stay as onlookers to what is happening to the brotherly Syrian people, and how much longer will we grant the Syrian regime one period after another so it can commit more massacres against its people?"
"At our meeting today I call for decisive measures, after the failure of the half-solutions," he said. "The Arab League should...open all channels of communication with the Syrian opposition and give all forms of support to it."
He did not specify if that support should include military aid. Western powers have shunned military action, despite widespread condemnation of the repression of the uprising.
In the besieged Syrian town of Homs, sporadic rocket and gunfire broke a respite in government bombardments of opposition-held neighbourhoods, killing at least four people, according to the activist Syrian Observatory for Human Rights.
A Syrian Red Crescent aid convoy had reached Homs and volunteers were distributing food, medical supplies, and blankets to thousands of people affected by the violence, the International Committee of the Red Cross said in a statement.
"The population, particularly the wounded and sick, are bearing the brunt of the violence," the ICRC chief in Damascus, Marianne Gasser, said. Aid workers were also helping thousands of people had fled to Bludan after fleeing from nearby Zabadani, 50 km north-west of Damascus, the Red Crescent said. Crowds attended the funerals of some of the 28 people killed in bombings of two military sites in the northern city of Aleppo on Friday - attacks the government cited as proof of its contention that it is fighting foreign-backed terrorists.
Sunday's meeting in Cairo opened with the resignation of the Sudanese general who led an Arab League observer mission to Syria in December, Mohammed al-Dabi. He had been a controversial figure because of his country's own poor human rights record. Ministers from Gulf states, which have been leading the drive to isolate Assad, met separately earlier and discussed recognising the opposition Syrian National Council as Syria's government. A Gulf source said they would propose Arab states each take such a step.
In Homs, shelling had eased during Saturday night and Sunday morning before Assad's forces renewed their rocket barrages. At least four people were killed, activists said.
Activist reports are hard to verify independently but form the basis for reporting the conflict as the Syrian government has restricted Western media access to much of the country.