An American woman who had her phone seized by border agents as she returned home to the United States is suing the country's border protection agency.

Rejhane Lazoja was stopped at Newark airport, New Jersey, after returning from a trip to Switzerland in February.

Her iPhone was seized by agents after she refused to unlock it for them.

The lawsuit alleges that border agents took a copy of the data on her smartphone and failed to say whether it had been deleted.

According to legal documents, US Customs and Border Protection (CBP) kept the phone for more than 120 days before returning it to Ms Lazoja.

She alleges the agency failed to confirm whether it had taken a copy of her data, and whether that data had been shared with any other agencies.

The smartphone contained photographs of Ms Lazoja "in a state of undress", as well as private messages from her lawyer.

"Ms Lazoja is a Muslim woman and wears a hijab (a headscarf) in accordance with her religious beliefs. Pursuant to her sincerely held beliefs, Ms Lazoja cannot be seen in a state of undress by men who are not family members," the documents state.

"Neither was there probable cause, nor a warrant [to search the phone]. Therefore, the search and seizure of Ms Lazoja's property violated her rights under the Fourth Amendment," the filing says.

The case will be heard in a New Jersey US district court.