UN chief Ban Ki-moon and the head of the Organization for the Prohibition of Chemical Weapons (OPCW) are concerned at the possibility that Syria may have chemical weapons, a UN spokesman said on Thursday."On Syria, the secretary-general and the director-general noted with concern the reports on the possible existence of chemical weapons in the country," UN spokesman Martin Nesirky told reporters. "Those concerns are entirely understandable.""The members of the Security Council call upon the Syrian authorities to grant (Amos) immediate and unhindered access," said the statement, which was read to reporters by British Ambassador Mark Lyall Grant.The statement also urged Damascus to grant "full and unimpeded access of humanitarian personnel to all populations in need of assistance."Syrian authorities did agree on Thursday to let Red Cross aid workers enter the Baba Amro district of Homs on Friday, a spokesman for the International Committee of the Red Cross (ICRC) said in Geneva.The move came as a three-week army siege of the neighborhood appeared to be ending on Thursday with the withdrawal of rebel forces."The Syrian Red Crescent and ICRC have received a green light from the authorities to go to Baba Amro tomorrow, Friday, in order to bring in much needed assistance including food and medical aid and to carry out evacuation operations," ICRC spokesman Hicham Hassan told Reuters.Syrian authorities also gave the ICRC "positive indications" on the agency's Feb. 21 request for a daily, two-hour ceasefire to deliver life-saving relief supplies to civilians, he said.The green light came as Syrian rebels left Baba Amro after a 26-day military siege aimed at crushing a symbol of the revolt against President Bashar Assad.Top US military officer, General Martin Dempsey, chairman of the US military's Joint Chiefs of Staff, said last week that Syria is "very capable," with a sophisticated, integrated air defense system and chemical and biological weapons.An 11-month government crackdown on pro-democracy protesters has left an estimated 7,500 civilians dead, according to the United Nations, and the outside world has proved powerless to halt the killing. Russia and China, who have twice used their vetoes to block any action by the UN Security Council against Syria, joined their fellow UNSC members on Thursday in expressing "deep disappointment" at Syria's failure to allow the UN humanitarian aid chief Valerie Amos to visit the country and urged that she be allowed in immediately.The 15 nations on the council also said in a unanimously agreed statement that they "deplore the rapidly deteriorating humanitarian situation" in Syria.