Syrian rebels have captured a key oilfield in eastern Deir Ezzor province, activists say, after a siege lasting several days.The Syrian Observatory for Human Rights group said al-Ward fell after fierce fighting, although the reports have not been independently confirmed.The Observatory said it was the first time the rebels had taken control of an oilfield.The news came as various opposition groups met for crucial talks in Qatar.SanctionsThe head of the UK-based Observatory, Rami Abdel Rahman, said: "Rebels in the Jaafar Tayyar Brigade took control of al-Ward oilfield, east of the town of Mayadin, after a siege that lasted several days.”The Observatory also cited witnesses, residents and activists as saying rebels had shot down a warplane that had been carrying out attacks on Mayadin.The Observatory is one of the most prominent organisations documenting and reporting incidents and casualties in the Syrian conflict. The group says its reports are impartial, though its information cannot be independently verified.Mr Rahman said some 40 troops guarding the oilfield had been killed, wounded or captured.One activist in the area, Omar Abu Leila, told Associated Press news agency the field was still working until shortly before it was captured.Al-Ward is said to be one of the most important oilfields in Deir Ezzor, which holds most of Syria’s energy reserves.Oil was the main source of hard currency for the government of President Bashar al-Assad until the European Union - which had bought 95% of Syria’s output - imposed sanctions last year.Oil exports fell to 7,500 tonnes in the first quarter of 2012 from 13,500 tonnes in the fourth quarter of 2011. Agencies