The Arab League tightened its sanctions against Syria early Thursday in cooperation with the international effort to force President Bashar al-Assad to end government violence against civilians or step down from power. 

The Cairo-based regional body added 17 high-ranking Syrian officials to a list banning travel to other Arab states, including on its "black list" Assad's brother, Maher al-Assad, according to the Egyptian MENA state news agency.

Maher al-Assad, considered the second most powerful man in Syria, is commander of the feared Syrian Republican Guard.

Also on the list were Syria's ministers of Defense and the Interior, as well as senior military and intelligence officials and Assad's millionaire cousin, Rami Makhlouf, head of the country's mobile phone network.

Nevertheless, more than a dozen people were reportedly killed on Wednesday as government troops continued their attacks on anti-government protesters.

The United Nations has estimated that close to 4,000 people have been killed in the developing civil war ignited by the "Arab Spring" since March, but activists and human rights organizations have claimed the figure to be much higher, at least 4,500 dead. Thousands more have been wounded, and even more have been arbitrarily detained, tortured and "disappeared," among them many children, according to the U.N. Human Rights Council, which has charged Assad with crimes against humanity committed by his troops. 

Turkey, which accounted for some $2.4 billion in trade with Syria last year according to its embassy in Damascus, also announced sanctions against the Assad regime this week.

Foreign Minister Ahmet Davutoglu told reporters in Ankara, "Every bullet fired, every bombed mosque has eliminated the legitimacy of the Syrian leadership and has widened the gap between us. Syria has squandered the last chance it was given."