US and Russian military officials have signed a memorandum of understanding that includes steps their pilots should take to avoid an inadvertent clash over Syria as they carry out separate air strikes against armed groups, the Pentagon said. The issue of aircraft safety started after Russia started bombing targets in Syria last month. Moscow says it is attacking the Islamic State of Iraq and the Levant (ISIL), but many of its air strikes have hit territory held by other armed groups fighting against Russia's Syrian ally, President Bashar al-Assad. Pentagon spokesman Peter Cook released few details about the memorandum, purportedly at the request of Moscow. Cook said that the Russians did not want the details put out publicly. "As you might expect, that is making it hard for people to judge whether the memorandum will be effective.” "Having said that, there is now a signed agreement, signed by US and Russian officials, which spells out the rules of the airspace over Syria." Cook did say that the memorandum included specific protocols for air crews to follow, plus the creation of a ground communications link between the two sides in the event air communications fail.