Washington (AFP) - Secretary of State John Kerry warned that the Republican presidential campaign has descended into "an embarrassment" that raises awkward questions abroad about the reliability of the United States.

Kerry said that everywhere he goes, every leader he meets asks about what is happening in America.

"They cannot believe it. I think it is fair to say that they're shocked. They don't know where it's taking the United States of America," Kerry said in an interview on the Sunday CBS news show "Face The Nation."

"It upsets people's sense of equilibrium about our steadiness, about our reliability, and to some degree I must say to you, some of the questions, the way they're posed to me, it's clear to me that what's happening is an embarrassment to our country."

Kerry was asked out the impact abroad of the Republican campaign with its calls for bans on Muslim immigrants, surveillance of Muslim neighborhoods and the return of waterboarding, an interrogation practice regarded as torture.

Frontrunner Donald Trump and his chief rival Ted Cruz have stepped up the anti-Muslim rhetoric since the suicide bombings in Brussels Tuesday that left 28 dead.

In an interview with The New York Times, Trump described his approach to foreign policy as "American first," borrowing the isolation slogan of Americans who wanted to keep the United States out of World War II.