DENVER — Negative opinions about Islam are on the rise, Mormons are viewed as Christian but different and Pope Benedict XVI trails his predecessor in popularity, a poll of Americans released Tuesday said.

The survey of 3,000 adults from Aug. 1-18 was conducted for the Pew Research Center and the Pew Forum on Religion and Public Life.

The number of Americans who say Islam has little or nothing in common with their own religion has spiked to 70 percent in the past two years from 59 percent, the poll found.

Another significant shift has taken place: In 2005, 36 percent of the public said Islam is more likely than other faiths to encourage violence among its believers. That number has risen to 45 percent.

Fifty-three percent of Americans viewed members of The Church of Jesus Christ of Latter-day Saints positively, while 27 percent viewed Mormons unfavorably.

A slim majority — 52 percent — said Mormonism is a Christian religion. Yet among non-Mormon believers, more than six in 10 said Mormonism and their own religion are very different.

"The lack of information about Muslims and Mormons has to be put in broader context," said John Greel, a Pew Forum senior fellow. "Many Americans know very little about the larger religious groups. And very many people know very little about their own religions."

Green said an open-ended question asking respondents for one-word descriptions of Islam and Mormonism produced interesting data because "whether that is the whole story or not, you get a sense of what their initial thinking is."

Overall, only 15 percent of respondents came up with a "positive word to describe Islam, and only 23 percent used a positive word to describe Mormonism. For both religions, a plurality of respondents who came up with a word offered negative terms.

The most often used word to describe Islam was devout or devoted, closely followed by fanatic or fanatical and different.

The word used most often to describe Mormonism was polygamy or bigamy, closely followed by family or family values and cult. The LDS Church banned polygamy more than a century ago.

The poll also found that Pope Benedict XVI has high approval ratings with the public two years into his papacy: 73 percent with an opinion view him favorably. Those numbers lag behind those of his more traveled predecessor, Pope John Paul II, who garnered 86 percent favorable ratings in 1996.

Benedict does not have the public's confidence in promoting good relations with other religions, the poll found. Nearly half who know something about the pope say he is only doing a fair or good job on that subject.

The poll's margin of error is plus or minus 2 percentage points.