THE United Nations Security Council extended for six weeks on Thursday the mandate of a UN mission charged with preparing an independence referendum in the disputed territory of Western Sahara. The vote came hours after Morocco signed a technical agreement which provides for the UN forces in the former Spanish colony to be armed in certain areas. In the referendum planned for December, Western Sahara will be asked to choose between independence, as demanded by the Polisario Front rebels and remaining part of Morocco, which annexed the territory in 1975.