The Syrian opposition has accused the Syrian regime and its allies of killing 29 people in attacks following the recently agreed cease-fire.In a letter sent to U.N. Secretary-General Ban Ki-moon on Sunday, the head of the opposition-linked Syrian High Negotiations Committee, Riyad Hijab, said that the Syrian regime, Russia, Iran and other foreign armed units were carrying out attacks on Syrians despite the agreement to cease hostilities that entered into force on Feb. 27.Hijab said regime forces had violated the deal in 25 regions held by opposition groups with 24 artillery shelling incidents and five ground operations. He also said that Russian warplanes supported regime forces by carrying out 26 air attacks, including attacks with cluster and vacuum bombs.The committee called on the U.N. and the Friends of Syria to take necessary measures to stop Russian violations.The cessation of hostilities agreement, brokered last week by the U.S. and Russia, came into effect at midnight last Saturday.Syria has remained locked in a civil war since early 2011 when Bashar Assad's regime cracked down on pro-democracy protests that had erupted as part of the Arab Spring uprisings.Since then, more than 250,000 people have been killed and more than 10 million displaced, according to U.N. figures.