The US embassy in Nigeria says that the authorities should investigate the deaths of protesters from a Shia Muslim sect during clashes with security forces earlier this week.
It wants the authorities to “take appropriate action to hold accountable those responsible for violations of Nigerian law”.
Members of the pro-Iran Islamic Movement of Nigeria (IMN) had been demanding the release of their leader Ibraheem Zakzaky, in custody for 34 months.
The army admits that three people were killed in clashes in Abuja on Monday, but the IMN says that dozens died.
On Wednesday, rights group Amnesty International said that 45 member of IMN died over two days of clashes.
The army, which fired live rounds on Monday at the protesters, said the deaths came after the demonstrators tried to overrun a checkpoint.
Amnesty said: “Video footage and eyewitness testimonies consistently show that the Nigerian military dispersed peaceful gatherings by firing live ammunition without warning, in clear violation of Nigerian and international law.”