As many as 70 Syrian army deserters were shot dead on Monday as they tried to flee their base in the north-western Idlib province, activists from the London-based Syrian Observatory for Human Rights said.
Witnesses said the defectors had been downed by machine gun fire as they attempted to leave and join the uprising against the government of President Bashar al-Assad.
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Activists say Monday's death toll across Syria could exceed 110, which would make it one of the deadliest days of the uprising, the BBC reported.
The violence comes the same day Syria signed an agreement to allow Arab League observers into the country.
The deal was reportedly signed by the deputy of Foreign Minister Walid al-Muallem at the Arab League's headquarters in Cairo.
Although Syria asked for some amendments to protect its sovereignty, Muallem said observers would be allowed to move freely under the protection of the Syrian government.
According to the BBC, Syria demanded that observers coordinate with local authorities, which the league accepted.
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The league's secretary general, Nabil al-Arabi, said the first foreign monitors would arrive in Syria "within two or three days."