Nato is set to approve the deployment of Patriot missile interceptors to defend Turkey’s border with Syria.A meeting of the 28-member alliance’s foreign ministers in Brussels follows a request from Turkey to boost its defences along the border.Nato officials have made clear such a move would be purely defensive.Earlier, US President Barack Obama warned Syrian President Bashar al-Assad he would face "consequences” if he uses chemical weapons against his people."The world is watching. The use of chemical weapons is and would be totally unacceptable,” said Mr Obama in a speech at the National Defense University in Washington."If you make the tragic mistake of using these weapons there will be consequences and you will be held accountable.”A Syrian official has insisted it would "never, under any circumstances” use such weapons, "if such weapons exist”.The missile deployment is likely to be approved, adds our correspondent, despite opposition from Russia, whose foreign minister is also attending Tuesday’s meeting in Brussels.But analysts say any deployment - possibly supplied by the US, Germany or the Netherlands - could take weeks.Syrian opposition fighters have reportedly made dramatic gains recently, and several government mortar shells - aimed at rebel targets close to the border - have landed across its 900-km (560-mile) border with Turkey.Ankara’s request for Nato to deploy the anti-missile batteries came after intelligence assessments that Damascus was contemplating using ballistic missiles, potentially armed with chemical warheads, reports say.Syria is believed to hold chemical weapons - including mustard gas and sarin, a highly toxic nerve agent - at dozens of sites around the country.