"The two leaders underscored the urgency of the situation, reiterated their deep concern about the Syrian government's use of violence against civilians and their belief that the Syrian people's legitimate demands for a transition to democracy should be met," the White House said in a readout of the call.
And US Secretary of State Hillary Clinton urged China and Russia to step up pressure on Assad.
"We want to see China take steps with us.... We want to see Russia cease selling arms to the Assad regime," Clinton said in an interview with CBS News.
Meanwhile, Syrian forces killed at least 16 people on Thursday, with the army storming more towns in pursuit of anti-regime protesters, defying Western calls for an end to violence.
The killings occurred soon after columns of tanks entered the town of Qusayr in the central province of Homs early on Thursday, sending residents fleeing into the fields, rights activists there said.
"Twelve people were killed by the end of military operations" in Qusayr, the Syrian Observatory for Human Rights said.
Earlier an activist told AFP in Nicosia that "security forces opened fire on residents who tried to flee to the Al-Basateen district."
Another activist said dozens of people had been wounded, adding that the army "has closed entrances to the town," while security forces made around "one hundred arrests", the Observatory reported.
Three other people were shot dead by security forces in the eastern oil hub of Deir Ezzor's Al-Matar neighbourhood and several houses were torched, the Britain-based rights group said.
Another person was killed in the coastal city of Lattakia where Assad's forces unleashed another wave of arrests.
Tanks, troop carriers and buses transporting security forces also sped into the town of Saraqeb in the northwestern Idlib province bordering Turkey soon after dawn Thursday, the Observatory reported.
"Shooting was heard soon afterwards," it said, adding that the forces were "raiding homes and making arrests, rounding up more than 100 people, including 35 children."
"Army troops are smashing the doors of shops owned by activists in search of them, and they have cut off electricity in the town," where anti-regime protests have been held nightly, said the Observatory.
Hundreds demonstrated late Thursday despite the heavy security presence, it added.
On Wednesday, 18 people were shot dead in the Baba Amro neighbourhood of the city of Homs with more than 100 wounded "some in critical condition," the group said.
It said residents were fleeing for safety while heavy machine-gun fire rattled Baba Amro well into Thursday morning.
The group also reported that security forces arrested 27 activists Wednesday in the town of Salamiyeh and nearby villages, in the province of Deir Ezzor, including prominent dissident and protest organiser Hassan Zahra.
Abdel Karim Rihawi, head of the Syrian League for the Defence of Human Rights since 2004 and a key source of information for international media, was arrested on Thursday, activists said.
The latest assaults come after UN Assistant Secretary General Oscar Fernandez-Taranco briefed the Security Council Wednesday about events in Syria in the week since the council called for an "immediate" halt to the violence.
Taranco, who spoke behind closed doors, was quoted as saying there had been no letup in the deaths of protesters while UN officials had met Syrian diplomats to try to get accurate information.
His briefing was "depressing and chilling," Britain's deputy UN ambassador Philip Parham said.
The United States has decided to call explicitly for Assad to step down, and the expected announcement could be made as early as Thursday, officials said Thursday.
"The United States is looking to explicitly call for Assad to step down. The timing of that is still in question," a US official told AFP on the condition of anonymity, while another US official said the call could come as early as Thursday.
Ignoring the international outrage, Assad pledged this week a relentless battle against "terrorist groups" Damascus says is fomenting a popular uprising across Syria.
Rights groups say more than 2,000 people have been killed in the crackdown on the protest movement, which first erupted in mid-March with calls for reform before demanding the fall of the regime over its bloody repression.
The Observatory said on its website that a total of 2,150 people have been confirmed dead since the protests began, including 1,744 civilians and 406 security forces.
That toll, it said, excludes "martyrs who fell in Hama since August 3."
Meanwhile Facebook group The Syrian Revolution 2011 urged Syrians to pursue anti-regime rallies throughout the holy Muslim fasting month of Ramadan which started August 1, saying "every day in Ramadan is a Friday."
Friday -- the weekly day of rest when key Muslim prayers are held -- has become a focal point of anti-regime protests in Syria, with hundreds of thousands pouring on to the streets each week to demonstrate.
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