The Iranian Navy imposed a naval blockade of Iraq, using its British-built frigates to stop and inspect any ships thought to be trading with Iraq. They operated with virtual impunity, as Iraqi pilots had little training in hitting naval targets. Some Iranian warships attacked tankers with ship-to-ship missiles, while others used their radars to guide land-based anti-ship missiles to their targets. Iran began to rely on its new Revolutionary Guard's navy, which used Boghammar speedboats fitted with rocket launchers and heavy machine guns. These speedboats would launch surprise attacks against tankers and cause substantial damage. Iran also used F-4 Phantoms II and helicopters to launch Maverick missiles and unguided rockets at tankers.