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"On Monday, June 5, at approximately 3:00 am, electricity to the main building of the US interests section in Havana was cut off," State Department spokesman Sean McCormack told a press conference.
"The US interests section building is the only building in the neighbourhood without electricity," he said.
Mr McCormack said that the United States had written to the Cuban government asking for power to be restored, and had also issued a request through the Swiss embassy, which represents US interests.
But "the power remains cut off and the US interests section is operating on generator power."
The spokesman said work was continuing at the US mission, including interviews with refugees. He said he believed the communist regime had cut power to punish Washington over its efforts to provide information to Cubans about human rights and other issues.
The move "has everything to do with their displeasure over some of the activities" at the US mission, he said.
Cuba had also been periodically "turning down the spigot on the water supply," the spokesman said.
He said Cuba's strategy would have no effect on the US mission's efforts.
"I would just say that the bullying tactics of the Castro regime aren't going to work."
Asked if the US planned to retaliate against the Cuban diplomatic mission in Washington, Mr McCormack joked that if the Cubans had paid their electricity bills on time to the local utility, "I think they're probably OK."
Cuban President Fidel Castro has been irked by a huge electronic billboard on the US mission's facade, which runs messages promoting democracy and human rights.
In a bid to block views of the billboard, Cuba inaugurated a "mountain of flags" outside the US mission building in February, featuring 138 black banners with white stars honouring Cuban victims of terrorism.