US Secretary of State Hillary Clinton has urged all countries to cut their political and economic ties with Syria.
Mrs Clinton's comments came as large anti-government protests continued, despite a harsh army crackdown.
Activists said at least 20 people died yesterday as protesters came under fire in towns and cities across the country.
Mrs Clinton said buying oil and gas from Syria and exporting arms there were giving President Bashar al-Assad ‘comfort in his brutality’.
Mrs Clinton said Syria would be better off without Mr Assad and called on nations that buy oil or sell arms to Syria to cut those ties.
'We urge those countries still buying Syrian oil or gas, those countries still sending Assad weapons, those countries whose political and economic support give him comfort in his brutality, to get on the right side of history,' she said.
Asked why the US had not called on Mr Assad to step down, Mrs Clinton said Washington had been 'very clear' in its statements about Assad's loss of legitimacy, and wanted other nations to add their voices.
On Wednesday, Washington imposed sanctions on Syria's largest bank and its biggest mobile telephone company, controlled by Assad's cousin Rami Makhlouf.
The next day, US Ambassador to Damascus Robert Ford said more sanctions would follow if the Syrian authorities did not halt the violence.
In addition to the Friday protests, nightly Ramadan prayers, or ‘tarawih’, which follow the breaking of the fast, have given more Syrians a focus for daily protest marches.
Defiant protest marches unfolded across the country yesterday despite a military crackdown that has intensified since the beginning of the fasting month of Ramadan.
These included protests in the cities of Hama and Deir al-Zor, both of which have been stormed in tank offensives launched by President Assad at the beginning of the Muslim holy month.
A doctor in Deir al-Zor reported that military intelligence agents fired live ammunition at protesters coming out of several mosques, killing three people.
The doctor, who did not want to be named, said at least 80 people had been killed in Deir al-Zor since Sunday, when President’s Assad's forces, backed by tanks, stormed the city.
The Syrian Observatory for Human Rights said security forces killed four protesters in the commercial hub of Aleppo and one man was killed by a sniper in the city of Homs.
Residents said four people were also killed in Hama, just days after the army completed a week-long assault on the city which became a symbol of defiance to Assad's rule after huge crowds gathered weekly to demand his overthrow.
Syrian state television said two members of the security forces were killed by gunmen in Douma, just outside Damascus.
Syrian authorities have barred most independent media, making it difficult to verify events on the ground in the unrest, among a series of popular revolts against repressive power elites across the Arab world this year.
President Assad's forces have intensified assaults on towns and cities across the country since the start of Ramadan nearly two weeks ago to try to subdue mounting dissent against the ruling family, despite threats of new US sanctions and calls from Turkey and fellow Arab states to end attacks on civilians.
