The Home Office is not confirming or denying whether any British workers have been involved.
But officials say they are investigating reports which suggest at least one Briton was taken when gunmen stormed a compound belonging to a Lebanese construction company in Nigeria.
ITV News reporter Sejal Karia talks from outside the Foreign Office:
More top news
Saffie Roussos, believed to be eight years old, was described as a "beautiful little girl".
The first victim of the Manchester bombing has been named as Georgina Callander, a student from Lancashire.
A 23-year-old man has been arrested in connection with the attack at Manchester Arena last night.