U.S. Agency for International Development administrator Rajiv Shah says the United States has led an aggressive effort to free American Alan Gross, who has been imprisoned in Cuba since 2009.

Testifying Tuesday before a Senate committee, Shah called Gross' detention "wrong,'' but said "the responsibility for his detention rests with Cuban authorities.''

Gross was working for USAID setting up an Internet network for Havana's Jewish community, when he was arrested and charged with illegally distributing communications equipment on the island. He was eventually tried and sentenced to 15 years in prison.

According to his attorney, Gross has begun a hunger strike in prison to object to what he says are "mistruths, deceptions, and inaction" by governments in Havana and Washington over his "arbitrary detention."

Gross called for U.S. President Barack Obama to personally intercede on his behalf.

His case may be complicated by last week's Associated Press report that USAID had financed a social network in Cuba between 2010 and 2012. The AP says the aim of the so-called "Cuban Twitter" was to stir political unrest and undermine the Caribbean island's communist government.

The network, dubbed ZunZuneo, reportedly drew tens of thousands of subscribers who were unaware it was backed by the U.S. government.

In his Senate testimony, Shah said Gross' work and ZunZuneo were "absolutely not'' covert. He said creating platforms to improve communication in Cuba and elsewhere is a "core part" of what USAID does.

Shah took over USAID after the ZunZuneo project had ended.