By 1941 the United States was taking an active part in the war, despite its nominal neutrality. In spring U-boats began their "wolf-pack" tactics which threatened to sever the trans- Atlantic supply line; Roosevelt extended the Pan-American Security Zone east almost as far as Iceland. The US Navy's "neutrality patrols" were anything but, as in practice their function was to report Axis ship and submarine sightings to the British and Canadian navies, and from April the US Navy began escorting Allied convoys from Canada as far as the "Mid-Atlantic Meeting Point" (MOMP) south of Iceland, where they handed off to the RN.