The U.S. ambassador to the United Nations on Wednesday expressed concern about the “escalating politically motivated violence” in Zimbabwe and urged a joint role by the world body and the African Union to resolve the election crisis. “We are gravely concerned about the escalating politically motivated violence perpetrated by [Zimbabwean] security forces and ruling party militias,” Ambassador Zalmay Khalilzad told a high-level meeting of the U.N. Security Council. Khalilzad said the United Nations must back efforts by southern African leaders to settle the crisis “through a joint mission with the African Union to ensure that … the will of the Zimbabwean people is upheld.” Zimbabwean President Robert Mugabe's security forces cracked down on unrest during a general strike, arresting many opposition supporters before the strike ended on Wednesday. “The [Harare] government and its supporters must desist immediately from violence and intimidation, act with restraint, respect human rights, and allow the electoral process to continue unfettered,” Khalilzad said. Opposition leader Morgan Tsvangirai contends he defeated Mugabe in the presidential vote last month, but the ruling party says neither candidate won a clear victory and insists a run-off election will be needed. Official results have not been released by electoral authorities. Khalilzad expressed support for U.N. Secretary-General Ban Ki-moon's call for international observers to monitor any second round of the Zimbabwe presidential vote. “If there is a second round of elections, they must be conducted in a fair and transparent manner, with international observers,” the secretary-general told the Security Council earlier on Wednesday.