On 4 April, reacting to the fierceness of the offensive, President Nixon authorized tactical airstrikes from the DMZ north to the 18th parallel, the southern panhandle of North Vietnam. This supply interdiction effort was the first systematic bombing carried out in North Vietnam proper since the end of Operation Rolling Thunder in November 1968. Airstrikes north of the 20th parallel were authorized on 5 April under the cover name Operation "Freedom Train". The first B-52 strike of the new operation was conducted on 10 April. President Nixon then decided to up the ante by targeting Hanoi and Haiphong. Between 1 May and 30 June, B-52s, fighter-bombers, and fixed-wing gunships had carried out 18,000 sorties over North Vietnam and suffered 29 aircraft losses.