LONDON (AP) — President Barack Obama is urging Britons to vote to stay in the European Union, saying the challenges in the world require allies to "stick together."

In an op-ed published in the Telegraph newspaper, Obama says Great Britain's presence in the EU "magnifies" Britain's influence and helps spread "British values."

The piece was published Thursday evening as Obama arrived in London for a three-day visit. Some have speculated that the trip is timed to boost the campaign against the so-called "Brexit" ahead of a June referendum. Not all Britons are welcoming Obama's opinion.

Obama writes he's offering his view "with the candour of a friend" and notes the decision will affect U.S. interests.

He writes, "The U.S. and the world need your outsized influence to continue - including within Europe."