Critics of the Iranian government expressed concern about political rights there. This week marks the second anniversary since authorities placed opposition leaders Mir Hossein Mousavi and Mehdi Karroubi under house arrest.
Both men challenged Iranian President Mahmoud Ahmadinejad in a 2009 election. Ahmadinejad's controversial victory sparked protests not seen since the Iranian revolution in 1979.
Burt called on the Iranian government to free opposition figures and assure citizens' rights to freedom of expression are protected.
"These actions demonstrate a cynical attempt to suppress opposition figures and their families in defiance of Iran's obligations under international law," Burt said in a statement.
Two of Mousavi's daughters and Karroubi's son were arrested recently.
Iran is to have presidential elections this year. Ahmadinejad is ineligible to run because of term limits and he's also apparently fallen out of favor with Iran's cleric rulers.
Ahmed Shaheed, U.N. special on the human rights situation in Iran, this week expressed concern about the continued isolation of Iranian opposition leaders.