Nigerian lawmakers are probing the activities of more than a dozen non-profit organisations and demanded they submit within a week tax and financial statements dating back a decade, a letter seen by Reuters showed, prompting accusations of "bullying".

The groups affected include some who were previously funded by the U.S. Agency for International Development (USAID), Washington's primary humanitarian aid agency whose work has been largely frozen by the Trump administration for 90 days pending assessments of their effectiveness.

A committee of Nigeria's House of Representatives sent out letters to groups involved in human rights and accountability work, including Transparency International Nigeria, informing them of the probe "with a view to unravelling their real identities, sources of funding and what they expend their monies on".

The letter said the investigation was a response to comments made last month by U.S. Congressman Scott Perry who, without providing evidence, said USAID had funneled money to various Islamist groups worldwide, including Boko Haram in Nigeria.

The U.S. ambassador to Nigeria, Richard Mills, rejected Perry's assertion regarding Boko Haram.

Nigerian lawmakers are demanding audited statements from the civil society groups, their sources of funding and how they spent their money between 2015 and 2024.

Some of the groups said the investigation was an excuse to undermine their work and amounted to a clampdown on free speech.

"On the basis of hearsay, just an unfounded allegation and lack of any proven evidence from either Nigerian security or Nigerian financial agencies, the national assembly will embark on this walk," Auwal Musa Rafsanjani, head of Transparency International in Nigeria, told Reuters.

Abiodun Baiyewu, the executive director of Global Rights, which is also being investigated, said of the investigation: "It is a brazen attempt at bullying the non-profit sector and not done in good faith."

House of Representatives spokesperson Akin Rotimi said the probe was not intended to harass non-profit organisations but that the allegations made by the U.S. Congressman were "too weighty to be dismissed".

"It is to gain a clearer understanding of how these funds have been managed over the years to determine whether there have been any lapses," he told Reuters.

Parliamentary committees in Nigeria can investigate any issue deemed of national interest and their reports if adopted by a majority of lawmakers can lead to the arrest or prosecution of groups or individuals.