Democratic presidential candidate Barack Obama on Friday was forced to denounce inflammatory remarks made by his pastor, who has railed against the United States and accused its leaders of bringing on the September 11 attacks by spreading terrorism.

Television networks have aired footage of his pastor's sermons likening the Democratic frontrunner to Jesus and declaring: "God damn America." In the sermons, Reverend Jeremiah Wright, who presided over Obama's marriage and provided the title of his book "Audacity of Hope", condemns what he describes as a systemic effort to keep black people in poverty.

"I categorically denounce any statement that disparages our great country or serves to divide us from our allies." Obama said on Friday. "I also believe that words that degrade individuals have no place in our public dialogue, whether it's on the campaign stump or in the pulpit. In sum, I reject outright the statements by Reverend Wright that are at issue."

The strong language and the accusations of racism have proved embarrassing to Obama, who has based his candidacy on a message of unity. In his sermons Wright reportedly refers to America as being under the influence of the Ku Klux Klan and describes black Republicans as sell-outs.

"The government gives them the drugs, builds bigger prisons, passes a three-strike law and then wants us to sing 'God Bless America'. No, no, no, God damn America, that's in the Bible for killing innocent people," he said in a 2003 sermon which was reviewed by ABC television.

"God damn America for treating our citizens as less than human. God damn America for as long as she acts like she is God and she is supreme."

Obama has been a member of the congregation of Wright's Trinity United Church of Christ in south side of Chicago for nearly 20 years, and he drew the title of his book from one of his pastor's sermons.

The pastor's sermons are available for sale at the church and both ABC and Fox News reviewed the addresses.

Obama wrote on the Huffington Post that he never heard Wright say any of the statements that are "so contrary to my own life and beliefs", but they have raised legitimate questions about the nature of his relationship with the pastor and the church.

He explained that he joined Wright's church, Trinity United Church of Christ in Chicago, nearly 20 years ago. He said he knew Wright as a former Marine and respected biblical scholar who lectured at seminaries across the country.

"Reverend Wright preached the gospel of Jesus, a gospel on which I base my life," he wrote. "... And the sermons I heard him preach always related to our obligation to love God and one another, to work on behalf of the poor and to seek justice at every turn."

He said Wright's controversial statements first came to his attention at the beginning of his presidential campaign last year, and he condemned them. Because of his ties to the 6,000-member congregation church - he and his wife were married there and their daughters baptised - Obama decided not to leave the church.

"With Reverend Wright's retirement and the ascension of my new pastor, Reverend Otis Moss, III, Michelle and I look forward to continuing a relationship with a church that has done so much good," Obama wrote.

In January, Wright spoke from the pulpit in praise of Obama's leadership and comparing him to Jesus's struggles under the Romans, according to Fox television. "Barack knows what it means living in a country and a culture that is controlled by rich white people," Wright said. "Hillary would never know that."

He went on: "Hillary ain't never been called a nigger. Hillary has never had a people defined as a non-person."

Wright also took issue with the idea that Bill Clinton had been a friend to African Americans. "Hillary is married to Bill, and Bill has been good to us. No he ain't! Bill did us, just like he did Monica Lewinsky. He was riding dirty."

On the Sunday immediately following 9/11, Wright described the attacks as retribution for the bombing of Hiroshima, America's policy in the Middle East and apartheid era South Africa. Obama has told reporters he was not in church on that Sunday.

However, it is the inversion of the line "God Bless America" that may expose Obama to the greatest criticism. Last month, his wife, Michelle, was criticised by rightwing blogs and indirectly by the Republican nominee, John McCain, for saying she had not been proud of being an American for most of her adult life.

In a meeting with Jewish leaders in Cleveland earlier this month, Obama compared Wright to an "old uncle" who said disagreeable things. He went on to add: "I suspect there are some of the people in this room who have heard relatives say some things that they don't agree with, including, on occasion, directed at African Americans."

The latest focus on Wright comes at a potentially volatile stage of the campaign with a hiatus of several weeks before the next primary in Pennsylvania on April 22.

The lull between major contests has led to a series of rows between the Clinton and Obama campaigns over race and gender. Last week, Samantha Power, a foreign policy adviser to Obama, was forced to resign after calling Clinton a "monster". This week, Geraldine Ferraro, who was the Democrats' vice-presidential candidate in 1984, was forced to stand down after claiming Obama would not be leading the race if he weren't black.

Nor are such culture wars exclusive to the Democratic race. McCain has come under pressure to repudiate a televangelist supporter, the Reverend Rod Parsley. Parsley, who is from Ohio, has spoken of a clash of civilisations between Islam and Christianity.