Cigarette manufacturers intensified nicotine concentrations in tobacco and changed cigarette designs to increase the number of puffs per cigarette, they reported.

The alterations made cigarettes more addictive, making it harder for smokers to quit.

The latest analysis corroborated earlier findings by the Massachusetts Department of Public Health. Both studies were based on data submitted to the department by major cigarette brands under a requirement that has been in force there since 1997.

All of the major brands and types of cigarettes -- regular, menthol, full-flavored, light, ultralight -- were affected, the researchers found. Camel and Doral were named as the brands with the greatest increases in nicotine yields, followed by Newport. Top-selling Marlboro reportedly delivered the same amount of nicotine in 2006 as in 1997.

Phillip Morris, which makes Marlboro cigarettes, said the data reflected random variations -- both upward and downward -- and questioned whether "relatively minor" changes would increase addictiveness.