The FAPE journalists' federation in Spain said on Tuesday that three reporters, Antonio Pampliega, Jose Manuel Lopez and Angel Sastre, had disappeared in Aleppo, where they were working.

The association's president, Elsa Gonzalez, told Spanish national television on Tuesday that the reporters had not been heard from for several days and emphasized that the three were "missing," not "kidnapped."

"We don't know [if they were together] doing their investigative reports in Aleppo, just that another three Spanish journalists were kidnapped in the same area nearly a year ago," Gonzalez said.

Angel Sastre's Twitter account showed he posted his last tweet on July 10.

Aleppo, earlier Syria's commercial hub, has been the center of violence since 2012, when President Bashar al-Assad's army and rebel forces began fighting for control in the city. The area is also vulnerable to attacks by the militants of the self-styled "Islamic State" and allied groups, known to have kidnapped and killed journalists.

Violence kills several civilians

At least 10 people were killed in Aleppo on Tuesday, after a missile struck the city. The British based Syrian Observatory for Human Rights blamed Assad's army for the attack and reported an increased number of casualties at 18, with more than 50 wounded. The surface-to-surface missile destroyed several houses in the area, the Observatory reported.

Meanwhile, Syrian government forces, backed by the Lebanese Hezbollah, were advancing into the mountain resort of Zabadani, which has been under attack for nearly three weeks.

"This places the city of Zabadani under the full siege of the Syrian army and our resistance fighters," Hezbollah said in a statement published by The Associated Press. The city lies about 12 kilometers (7.5 miles) from the border crossing on the Damascus-Beirut highway and is therefore of strategic importance.

The Shiite fighters of Hezbollah, allegedly backed by Iran, have been trying to eliminate rebel fighters between Syria's capital Damascus and Lebanon.

Conflict in Syria swiftly turned violent after mostly peaceful protests demanding the resignation of President Bashar Assad were launched in 2011. More than 220,000 people have died in the civil war in the last four years. Rebel forces and Assad's loyalists have been since fighting for control over a country; more recently, parts of Syria have been overrun by the fighters from the group calling itself "Islamic State."

mg/msh (AP, Reuters, dpa)