"People are dying in Syria and there are residential neighborhoods under siege," the Turkish prime minister said.
Violence continued in Syria even as the influential regional leader demanded that the United Nations "Security Council must act in order to stop the massacres."
Gunfire and explosions rocked the country, with blasts killing at least nine people in the Daraa governorate, according to the opposition Local Coordination Committees in Syria.
In the suburbs of Damascus, explosions and gunfire erupted in the morning, the opposition activists said. Snipers targeted moving objects as security forces deployed tanks at various checkpoints, according to the group.
Speaking in Istanbul, U.S. Secretary of State Hillary Clinton announced an additional $12 million in U.S. aid to the Syrian opposition, doubling the amount of American money pledged for humanitarian aid including field hospitals and medical training.
In a strongly-worded speech, Clinton accused al-Assad's troops of launching new assaults, tightening their siege of residential neighborhoods and crushing dozens of peaceful protests.
She said al-Assad was "adding to its long list of broken promises" by failing to implement a peace plan put forward by former U.N. Seccretary General Kofi Annan.
The Friends of Syria group will work on coordinating sanctions against Syria "to isolate this regime, cut off its funds, and squeeze its ability to wage war on its own people," she said.
She said the message was clear: "Stop killing your fellow citizens or you will face serious consequences."
There was no mention of authorizing airstrikes targeting Syrian forces as some, such as U.S. Sen. John McCain, have requested.
Syrian State TV carried some of the Istanbul speeches live, labeling the Friends of the Syrian People meeting the "Conference of the enemies of Syria."
"It is great that the conference is taking place on April 1 because it is April Fools' Day," the Syrian news anchor said, accusing the attendees of serving Israeli interests and Erdogan of carrying out Clinton's bidding.
Small pro- and anti-regime demonstrations took place outside the conference center in Turkey, closely monitored by Turkish riot police. A larger anti-Assad demonstration was scheduled for later on Sunday.
Syrians started protests against the regime a year ago, and were met with a fierce government crackdown.
The United Nations estimates at least 1 million have been affected and more than 9,000 have died since the unrest began. Opposition activists put the death toll at more than 10,000 people.
CNN cannot independently confirm reports from inside Syria because the government severely restricts access by international journalists.
Protesters have remained defiant in the uprising while soldiers have defected and taken up arms against former comrades, hoping to drive out the embattled leader through force.
However, there has been no main opposition leader or group over the protesters, and there is little indication of a coherent plan for Syria should al-Assad and his regime fall.
The United States has urged the opposition to unite and highlighted the difficulty in achieving that goal. A senior U.S. official said there will be no sweeping efforts if the opposition is disjointed.
"They are all over the map, depending on whom you talk on any given day," the U.S. official said. "It's hard to think of what we can do going forward, when there is no credible alternative."
Opposition officials are working to change that, and have taken two important steps, according to the State Department.
"One, they've adopted a -- what they're calling, a national pact. And the idea here is to unite the Syrian opposition around a concept and a vision going forward," a State Department official said Saturday.
Burhan Ghalioun, the leader of the opposition Syrian National Council, Sunday challenged the international community to commit as deeply to getting rid of al-Assad as the opposition has done.
"We need the Arab and international positions to be at the same level," he said.
The Friends of Syria conference comes after Annan urged the government to lay down its weapons as part of a peace plan to help end the yearlong crisis.
Last week, Al-Assad pledged to implement the peace plan brokered by Annan and vowed to "spare no effort" to ensure its success. However, he demanded that those battling his regime pledge to stop their violence too.
Annan's terms of the peace plan included an end to all violence by the government and opposition, the delivery of timely humanitarian aid, the release of arbitrarily detained people, freedom of movement for journalists, respect for peaceful demonstrations and freedom of association.
