ISTANBUL - A second group of rebel fighters trained in Turkey by the US-led coalition could be deployed to Syria within weeks as part of a campaign to push Islamic State insurgents away from the border, diplomatic sources told Reuters on Friday.The United States and Turkey plan to provide air cover for what Washington judges to be moderate Syrian rebels, in a joint operation to flush Islamic State from a rectangle of border territory roughly 80 km (50 miles) long. US jets have already begun air strikes from Turkish bases in advance of the campaign.Diplomats familiar with the plans say cutting Islamic State's access to the Turkish border, across which the radical insurgent group has been able to bring foreign fighters and supplies, could be a game-changer.But the plans have been beset by difficulties.The al Qaeda-linked Nusra Front said late last month it had detained some of a first group of less than 60 US-trained rebels in northern Syria weeks after they were deployed, and warned others to abandon the program.Their capture highlighted the fragility of the US-led efforts to train and equip thousands of screened members of the Syrian opposition over the next three years, hoping to help them defend communities against Islamic State."Although there has been some skepticism about it, it is far too early to write off this program. Massive resources have been invested in this to make it work and we think it will work in the end," one of the diplomatic sources said.The second group of rebels was currently in training in Turkey with US and British military instructors and would be deployed once that was completed in the next few weeks. Where exactly in Syria they were sent would depend on "the latest battlefield dynamics", the source said.Around 1,000 fighters in total were expected to be deployed to Syria by the end of this year, the source added.