The International Commission of Jurists (ICJ), a Geneva-based panel of senior lawyers and judges from around the world, said attacks by security forces on civilians amounted to crimes under international law.
The group said it had received accounts of the crackdown from lawyers and human rights defenders within Syria. Syria has barred foreign journalists since launching its crackdown on protests, making independent accounts hard to obtain.
"More than 700 people have reportedly been unlawfully killed and hundreds subjected to enforced disappearances since the Syrian authorities began their crackdown on 15 March in Deraa, Homs, Banias and other cities," the ICJ said in a statement.
"The ICJ continues to receive credible reports indicating that bodies have been left in the streets for days and the injured blocked from accessing medical facilities," it said.
"In addition, a number of people trying to leave the country have been tracked down and prevented from doing so by security services at the borders."
Syrian forces spread through southern towns on Thursday and tightened their grip on two other cities, broadening their military crackdown on protests against President Bashar al-Assad`s government.
"The Syrian government is using armed forces and tanks to indiscriminately shell cities, effectively imposing a siege on the population, in order to counter largely peaceful protests," Wilder Tayler, ICJ Secretary-General, said.
The U.N. Security Council should consider options, including invoking Chapter 7 of the U.N. charter, he said, referring to a document which can be used to authorise measures including economic and diplomatic sanctions as well as military action. (*)
Editor: Kunto Wibisono
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