Syria opposition meeting discusses appointing rival PM 18 March 2013 Last updated at 18:44 Syria's main opposition coalition is meeting in Istanbul to discuss the appointment of a rival prime minister for rebel-held parts of the country. An alternative government could then be formed to oversee the provision of services in opposition held-areas. Gen Selim Idriss, chief of staff of the rebel Free Syrian Army (FSA), said at the meeting that he supported the move. Meanwhile, the US condemned a Syrian airstrike on the border with Lebanon, calling it a "significant escalation." Reports from Lebanon said that Syrian aircraft had fired four rockets at the border between the two countries, near the Lebanese town of Arsal. There were no casualties from the raid. Lebanese officials had earlier said it was not clear whether the rockets had landed inside Lebanese territory. "These kinds of violations of sovereignty are absolutely unacceptable," US State Department spokeswoman Victoria Nuland told reporters. FSA backing Large swathes of northern Syria have been seized by rebels from government control in recent months. However, reports from those areas say that basic supplies such as electricity and water are limited. Rebel-held areas are currently administered by a patchwork of local councils and armed groups who have been running some institutions, such as courts and prisons. Gen Idriss told reporters that the FSA would work "under the umbrella" of any new government. "We demand an interim government for all Syrian territory, one that would be the only legitimate government in the country," Gen Idriss told the AFP news agency. Any institutions not following this government would be considered to be acting illegitimately and would be prosecuted. Also on Monday, the US said it would not stand in the way of other countries arming Syrian rebels. Last week France and the UK said they supported lifting the EU arms embargo on Syria to allow weapons to reach anti-government forces, citing guarantees from rebels that arms would not fall into the wrong hands. However, other EU countries have expressed scepticism over any such move. The embargo is expected to be discussed at a meeting of EU foreign ministers later this week and a vote is due in May. US Secretary of State John Kerry told reporters on Monday: "The United States does not stand in the way of other countries that made a decision to provide arms, whether it's France, or Britain or others." Last week saw the second anniversary of the Syrian uprising against the rule of President Bashar al-Assad, which initially began as a wave of peaceful protests but which is now often described as a civil war. An estimated 70,000 people have been killed and more than one million people have fled Syria since the uprising began, with thousands more displaced inside the country.