BEIRUT — The European Union (EU) escalated the pressure on Syria's government Friday by banning all imports of Syrian oil in response to its violent suppression of the nearly six-month uprising.

It was the most punitive action by the EU over the crackdown in Syria, which sells nearly all its oil to Europe and whose government relies heavily on that export income.

The new sanctions, which take effect Saturday, came as anti-government protesters in Syria marched and demonstrated in a number of cities and towns, including Damascus suburbs, despite a heavy presence of police officers and troops that in some cases outnumbered the protesters.

Activists and witnesses said 13 people were killed in Friday's protests, held under the slogan "Death Rather Than Humiliation," reflecting a growing frustration among activists that their largely peaceful gatherings have failed to crack the government.

The EU, like the United States, has already prohibited Europeans from doing business with top Syrian officials, including President Bashar Assad and his close aides, in an attempt to pressure him to halt the crackdown. The uprising represents the most serious challenge to his family's 40 years of autocratic rule.

Syria produces only about 400,000 barrels of oil a day, less than 1 percent of global production, and exports about 150,000 barrels a day, 95 percent of it to Europe. The exports provide about 25 percent of Syria's income and are an important source of cash for the government, which has raised subsidies on fuel and food and increased government salaries to try to ease the unrest.

Under the new prohibition, no member of the 27-member EU can purchase, import or transport oil and other petroleum products from Syria or enter into financial or insurance services for such transactions, as of Saturday, though existing contracts can be fulfilled until Nov. 15.

The EU also expanded its list of about 50 Syrians targeted by an assets freeze and travel ban, adding four Syrian businessmen accused of financing the government, diplomats said. Three companies were added to an existing blacklist of eight firms in Syria and Iran.

Even Assad's closest foreign supporters, Russia and Iran, have urged him to take steps that would mollify the protesters, who have been characterized by his government as terrorists and thugs under the influence of hostile foreign powers.

More than 2,200 people have been killed in Assad's suppression of the revolt that began in March, according to the United Nations.

Material from The Associated Press is included in this report.