She speaks with an English accent.
Actually, she speaks with an Indian English accent which is subtly different.
As for why she speaks this way, we don't know as little of her backstory is known.
It likely that she speaks this was because that is the way her family spoke in her expatriate community in the U.S. and, in addition, spending significant amounts of time in New Delhi where speaking thusly would be an advantage to an agent.
OUT OF UNIVERSE
That's Nagra's natural Leicestershire (UK Midlands) accent (influenced by her heritage) and it's likely that she just can't do a good enough American accent so they adjusted her character accordingly.
That said, it is not unusual, in my experience, to hear what are often perceived as "English" accents in the US from natural born US citizens... usually New England or East Coast accents which harken back to the earliest days of the US.