UNITED NATIONS (Reuters) - The size and sophistication of some recent bomb attacks in Syria suggest that "established terrorist groups" may have been behind them, the U.N. chief said on Friday, in a letter in which he urged states not to supply arms to government or rebel forces.
"The overall situation in Syria remains extremely serious and there has been only small progress on some issues," U.N. Secretary-General Ban Ki-moon told the Security Council in the letter on the 14-month-old conflict between government forces and increasingly militarized opposition.
"There is a continuing crisis on the ground, characterized by regular violence, deteriorating humanitarian conditions, human rights violations and continued political confrontation," he said in the letter, which was obtained by Reuters.
Ban said the unarmed U.N. observer force in Syria (UNSMIS), which is being deployed to monitor a cease-fire plan that has yet to take hold, noted that "significant parts of some cities appear to be under the de facto control of opposition elements.
"There is an overall atmosphere of tension, mistrust and fear," Ban said.
"The Syrian army has not ceased the use of, or pulled back, their heavy weapons in many areas," he said. "On several occasions, UNSMIS has heard the sound, or seen evidence, of shelling in population centers."
Ban said there has been an increase in the number of bombings in Syria, above all in Damascus, Hama, Aleppo, Idlib and Deir al-Zor.
"The sophistication and size of the bombs point to a high level of expertise, which may indicate the involvement of established terrorist groups," he said, without specifying to which groups he was referring.
Ban also warned countries against providing either side with military aid.
"Those who may contemplate supporting any side with weapons, military training or other military assistance, must reconsider such options to enable a sustained cessation of violence," he said.
A Russian cargo ship loaded with weapons is en route to Syria and due to arrive at a Syrian port this weekend, Al Arabiya television said in a report that Western diplomats in New York described on Friday as credible.
(Aditional reporting by Michelle Nichols; Editing by Anthony Boadle and Stacey Joyce)