ABC News' Kirit Radia reports: The US State Department says it is "deeply troubled" by reports of violence and arrests of political opposition following last Friday's presidential elections in Iran. Over the weekend and again today, reformist challenger Mir Hossein Mousavi rallied supporters in the streets, alleging voter fraud was to blame for his loss to Mahmoud Ahmadinejad, the hardline incumbent president of Iran. Reports say gunfire from pro-government militia killed at least one protestor today, though most riot police watched the largely peaceful demonstrations quietly. The Associated Press reports that a group of demonstrators lit a pro-government militia compound on fire. They were fired on from the roof as they tried to storm the building. "We are deeply troubled by the reports of violence, arrests and possible voting irregularities" in the wake of last week's elections in Iran," State Department spokesman Ian Kelly told reporters today, adding that "we do have doubts about the election results." On Saturday, Secretary of State Hillary Clinton "We obviously hope that the outcome reflects the genuine will and desire of the Iranian people." "As the president indicated last week, the enthusiasm and robust debate these elections engendered captured the attention of the world. And the essential right of people, to express themselves peacefully, needs to be respected. The international community remains committed to seeing Iran living up to its international responsibilities, and we will continue to use all avenues to try to convince Iran to meet its international obligations," Kelly added today. Kelly said the US, which has no diplomatic presence in Iran, is unable to judge whether the election was credible or not. "We are in a position of still assessing what went on. And it's difficult to assess, because there weren't any international monitors at the elections," he said. Asked if the US was encouraged by statements that Iran's governing body would conduct an investigation into allegations of voter fraud, Kelly replied only: "Iran needs to take these allegations of misconduct or of election irregularities seriously." The Obama administration has said it wants to engage in dialogue with Tehran. Asked if the US was now recalibrating those efforts, Kelly said the US would continue to pursue multilateral avenues to rid Iran of its nuclear capability. "We have expressed our willingness to sit down with them," Kelly said. "We'll look at all kinds of different avenues to get Iran to live up to its international obligations."