Before Syria's civil war, now in its third year, the shrine attracted tens of thousands of Shiite pilgrims from around the world. Last year, rebels kidnapped Iranian pilgrims visiting the area, accusing them of being spies. The pilgrims were later released.
In Lebanon, Hezbollah leader Sheik Hassan Nasrallah has called it "a duty" to protect the shrine, saying that if Syrian rebels destroyed it, that would ignite a sectarian war with no end.
Also Friday, pro-government Kurdish fighters battled al-Qaida-linked rebels in northeastern Syria, the latest in clashes that have killed more than 40 on both sides this week, activists said.
The Kurdish forces, which back Assad, have fought rebels from radical Islamic groups in the northeastern province of Hassakeh and the northern region of Aleppo for months now.
Fighting erupted again on Tuesday and the dead since then have included 15 Kurdish fighters of the pro-government Kurdish Democratic Union Party, or PYD, as well as 28 al-Qaida-linked fighters from the Jabhat al-Nusra or Nusra Front and the Islamic State of Iraq and the Levant, the Britain-based Observatory said.
The Kurdish militiamen captured the oil-rich area of Suweidiyeh and also the town of Ras al-Ayn near the border with Turkey, the Observatory said. It added that Friday's fighting focused mostly on towns and villages near Ras al-Ayn.
Kurds, the largest ethnic minority in Syria, make up more than 10 percent of the country's 23 million people and have seen their loyalties split in the conflict between pro- and anti-Assad groups. The minority is centered in the poor northeastern regions of Hassakeh and Qamishli, wedged in between the borders of Turkey and Iraq. The capital, Damascus, and Syria's largest city, Aleppo, also have several predominantly Kurdish neighborhoods.
In other developments, authorities in Damascus complied with a rebel demand and released several women prisoners, Lebanese officials said Friday.
The release was expected to set the stage for the freeing of several Lebanese Shiite pilgrims held by Syrian rebels since they were abducted in May 2012.
Lebanese security officials, speaking on condition of anonymity in line with regulations, said the women were released on Thursday. The Observatory said 23 women were freed, though there was no confirmation from Damascus.
It was unclear when or why the women were detained. There are tens of thousands of prisoners in Syrian jails, including many political prisoners and Assad opponents.
Lebanese officials have been shuttling between Syria and Turkey to try to mediate the pilgrims' release. In January, rebels freed 48 Iranians in exchange for more than 2,000 prisoners held by Syrian authorities.
More than 93,000 people have been killed since the Syrian conflict started in March 2011 as largely peaceful protests against Assad's rule. The crisis escalated into a civil war after some opposition supporters took up arms to fight a brutal government crackdown on dissent.
Also Friday, Lebanese military prosecutors filed charges against six members of the Nusra Front, accusing them of having weapons and explosive devices with the aim of "carrying out terrorist attacks" in Lebanon, the state-run National News Agency said. If convicted, they could face the death sentence, the report said.
Syria's civil war has spilled over to Lebanon on several occasions in the past months, killing scores. Many Lebanese Sunnis support the overwhelmingly Sunni uprising against Assad, while Shiites generally back Hezbollah and the Syrian regime.
The British government said Friday it banned effective immediately the Nusra Front, a move making membership in the group a criminal offense. The Home Office said it added the group to the government's list of outlawed organizations, a U.K.-wide roster of foreign and domestic terror groups.
Many Western governments have expressed concern that the Syrian conflict is serving as an incubator for Islamist terror. Last week senior British lawmakers warned that jihadists in Syria "currently represent the most worrying emerging terrorist threat to the U.K. and the West."
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Associated Press writer Zeina Karam contributed to this report.