"We think there ought to be a recount," Kerry told the House Foreign Affairs Committee on Capitol Hill, but added, "I don't know whether it's going to happen."
Kerry also told the House panel he had not yet determined if Washington would recognize Maduro's victory as legitimate.
"That evaluation has to be made and I haven't made it," he said.
"Obviously, if there are huge irregularities, we are going to have serious questions about the viability of that government," Kerry said.
Maduro, the late President Hugo Chavez's hand-picked successor, responded at a news conference in the capital Caracas that Kerry should steer clear of Venezuelan politics.
"It's obscene the U.S. intervention in the internal affairs of Venezuela," Maduro said at the Miraflores presidential palace.
"Take your eyes off Venezuela, John Kerry. Get out of here. Enough interventionism," he said.
Of the possibility Washington might not recognize his election, Maduro said: "We don't care about your recognition. We have decided to be free and we will be free and independent, with you or without you."
Venezuela's government Monday declared Maduro won Sunday's special election with 50.8 percent of the vote and said opposition candidate Henrique Capriles took 49 percent -- a separation of some 262,000 votes out of 14.9 million votes cast.
Capriles -- a 40-year-old law school graduate and governor of Miranda, Venezuela's second-most-populous state -- called Maduro's win "illegitimate," citing thousands of irregularities, and formally demanded a recount of all votes Wednesday, a call also made by the Organization of American States.
Maduro -- a 50-year-old former bus driver and union activist with close connections to the Castro brothers in Cuba -- initially said, after declaring victory Sunday night, he would welcome a recount.
"We'll do it. We are not afraid," he said.
But he reversed himself Monday, saying there would be no recount, and the National Electoral Council proclaimed Maduro the winner.
The council said the result was "irreversible."
Luisa Estella Morales, president of the Supreme Tribunal of Justice, Venezuela's highest court, affirmed the council's declaration Wednesday.
Morales, who was appointed to her post by the National Assembly, dominated by the ruling United Socialist Party, added a recount was impossible to do because the Venezuelan electoral system "is absolutely automatic."
She called Capriles' recount demand "a deceit of the people" that sought to destabilize the country.
At least eight people have died and 72 have been wounded in protests that have split the oil-rich country since the poll.
An opposition demonstration in downtown Caracas was canceled Wednesday after Maduro threatened to use "a hard hand" against protesters.
In Washington, White House spokesman Jay Carney urged all sides to avoid violence and other measures "that could raise tensions."
"We call on the Venezuelan government to respect the rights of Venezuelan citizens to peaceful assembly and free speech," Carney said in a statement.