The collapse of Portugal's Estado Novo government following the 1974 Carnation Revolution suspended all Portuguese military activity in Africa and the brokering of a ceasefire pending negotiations for Angolan independence. Encouraged by the Organisation of African Unity, Holden Roberto, Jonas Savimbi, and MPLA chairman Agostinho Neto met in Mombasa in early January 1975 and agreed to form a coalition government. This was ratified by the Alvor Agreement later that month, which called for general elections and set the country's independence date for 11 November 1975. All three factions, however, followed up on the ceasefire by taking advantage of the gradual Portuguese withdrawal to seize various strategic positions, acquire more arms, and enlarge their militant forces. The rapid influx of weapons from numerous external sources, especially the Soviet Union and the United States, as well as the escalation of tensions between the nationalist parties, fueled a new outbreak of hostilities. With tacit American and Zairean support the FNLA began massing large numbers of troops in northern Angola in an attempt to gain military superiority. Meanwhile, the MPLA began securing control of Luanda, a traditional Ambundu stronghold. Sporadic violence broke out in Luanda over the next few months after the FNLA attacked MPLA forces in March 1975. The fighting intensified with street clashes in April and May, and UNITA became involved after over two hundred of its members were massacred by an MPLA contingent that June. An upswing in Soviet arms shipments to the MPLA influenced a decision by the Central Intelligence Agency to likewise provide substantial covert aid to the FNLA and UNITA.