A resolution passed this year by the U.N. Security Council calls on the Yemeni government to take steps needed for general elections in 2014. Last year, former Yemeni President Ali Abdullah stepped down following lengthy demonstrations and his vice president, Abdu Rabbo Mansour Hadi, won a one-man contest to take his place in February.
Gunmen loyal to the former president attacked an administration building in Sanaa this week, leaving at least four people dead. The attack followed a decision by Hadi to reshuffle the military and reduce the number of units under the command of the former president's son.
Victoria Nuland, a spokeswoman for the State Department, said "dead-enders from the ancient regime" were throwing up roadblocks to Hadi's reform agenda.
Washington has stood by Yemen as part of a regional effort to keep al-Qaida influence at bay. Combating militants loyal to al-Qaida is among the many factors complicating Yemeni transition.
Nuland said Washington was calling on both sides to show restraint but said respect for Hadi's reforms was a key part of U.S. policy.
"What we are doing is trying to strengthen the regime of President Hadi, trying to work with him on these transition plans that he's putting in place," she said.