Query: The diplomatic agreement between South Africa, Angola, and Cuba was observed by the USSR and what other country?
Context: In 1978 the UN Security Council passed UN Resolution 435 which planned a transition toward independence for Namibia. Attempts to persuade South Africa to agree to the plan's implementation were not successful until 1988 when the transition to independence finally started under a diplomatic agreement between South Africa, Angola and Cuba, with the USSR and the USA as observers, under which South Africa agreed to withdraw and demobilise its forces in Namibia. As a result, Cuba agreed to pull back its troops in southern Angola sent to support the MPLA in its war for control of Angola with UNITA.