Britain's membership of the European Union magnifies its role on the global stage, U.S. Secretary of State John Kerry said on Tuesday, arguing that both Britain and the bloc were stronger when working together.

"A strong and united Europe that includes Great Britain is a stronger Europe and a stronger partner," Kerry told reporters in London just a few weeks after President Barack Obama visited the capital to urge Britons to stay in the EU.

"President Obama and I are both convinced that there is a great strength for Great Britain to be able to continue its role of leadership. We believe that membership magnifies Britain's role."

Britons vote in a referendum on whether to stay in the 28-member bloc on June 23, a decision with trade, investment, defence and political ramifications that stretch far beyond Britain's borders.

With just weeks to go, and with Britons evenly split over the issue according to polls, politicians from around the world have started weighing in with their opinions.

President Obama made some of the strongest remarks when he visited Britain in April, saying Britain would be at the "back of the queue" for a U.S. trade deal if the country left the EU.

In contrast Republican presidential candidate Donald Trump said last week he thought Britain would be better off outside the bloc.

Kerry, in Europe this week for meetings with counterparts, said he was not seeking to interfere in the vote, but said the U.S. had an interest in the outcome because it worked so closely with Britain.

"It's a partner and our special relationship plays out in many ways," he said. "But we respect the choice of the voters and they will listen to a broad array of thoughts. Ours is one opinion."