US Republican presidential candidate John McCain, who once sang in jest about bombing Iran, has reacted to a report of rising US cigarette exports to the country by saying it may be "a way of killing 'em."

Senator McCain, known for acerbic comments and for sometimes firing verbally from the hip, was responding to a report that US exports to Iran rose tenfold during President George W Bush's term in office despite hostility between the two states.

A rise in cigarette sales was a big part of that, according to an Associated Press analysis of seven years of US trade figures.

"Maybe that's a way of killing 'em," Senator McCain said to reporters during a campaign stop in Pittsburgh. "I meant that as a joke, as a person who hasn't had a cigarette in 28 years, 29 years," he added, laughing.

He declined further comment on the report.

At a campaign meeting in South Carolina last year the Arizona senator, asked if there is a plan to attack Iran, began his answer with a variation on the lyrics of a well-known pop song, Barbara Ann.

"You know that old Beach Boys song, Bomb Iran?" he said, then sang "Bomb bomb bomb, bomb bomb Iran" before discussing what he considered Iran's serious threat to Israel and international security.

Tension is high between the two countries over Iran's nuclear program, which Washington says is aimed at making an atomic bomb but Tehran says is for generating energy. There has been media speculation of a possible US or Israeli military strike against Iran's nuclear facilities.