A trio of watchdog groups has asked the US attorney for the Southern District of New York to investigate whether President Trump has received payments or other benefits from foreign governments through his business interests in violation of an obscure clause in the US Constitution.

The request, sent by letter Wednesday morning to US Attorney Preet Bharara, is a novel strategy by ethics critics who have been pressing Trump to comply with the Constitution's "emoluments clause," which prohibits top officials from receiving payments or favours from foreign governments. Trump's business empire stretches across the globe. The letter was sent six weeks after one of the groups filed a lawsuit in federal district court making a similar claim.

The letter notes that China recently granted Trump a trademark for building construction services. It also points out that tenants of Trump Tower in New York include a Chinese bank and the Abu Dhabi Tourism and Culture Authority and that the Trump hotel in Washington has been courting business from foreign diplomats. The document notes that Trump's business entities have plans to build luxury hotels in China and that it is also engaged in real estate and licensing projects in India, Indonesia, Turkey, Scotland Taiwan Saudi Arabia and the Philippines.

Trump has said he addressed ethics concerns by turning over management of his business to his sons, who've said they have separated the family business from their relationship with their father and the US government.