U.S.-backed forces have surrounded the northern Syrian city of Raqqa, the Islamic State group's so-called capital, dealing a symbolic blow to the extremist group as it continues to lose territory.However, human rights advocates are sounding the alarm about civilians still inside the besieged city, which ISIS seized in 2014.U.N. human rights chief Zeid Ra'ad Al Hussein said Wednesday that "up to 100,000 civilians are effectively trapped as the air and ground offensive intensifies."The independent Britain-based Syrian Observatory for Human Rights reported today that the U.S.-backed Syrian Democratic Forces have completely encircled the city after making gains south of Raqqa. "IS has no other choice now but to surrender or fight to the end," Observatory director Rami Abdurrahman told The Associated Press.The Observatory added that Islamic State militants launched a counteroffensive later in the day in a different part of the city, resulting in intense fighting and casualties on both sides.Col. Ryan Dillon, a spokesman for the U.S.-led coalition against ISIS, said in a tweet that the SDF "now control all high speed avenues of approach into #Raqqah from the south." The SDF, which is made up of Kurdish and Arab troops, initially focused on the north, east and west of the city, as we reported.Dillon said the Islamic State fighters, "abandoned by their leadership, are being pressured by the #SDF from multiple axes around the city."Hussein, the U.N. rights chief, said that the U.N. has documented at least 173 civilians killed in Raqqa this month — and he believes the actual toll "may be much higher."The SDF has encouraged civilians to flee from Raqqa. Many have. But the statement from Hussein's office said that fleeing also has proved extremely dangerous: