WASHINGTON - Former US secretary of state Colin Powell crossed party lines Sunday to endorse Senator Barack Obama for president as a candidate who was offering a "calm, patient, intellectual, steady approach" to the country's problems. The endorsement, on the NBC public affairs programme "Meet the Press", was a major blow to Senator John McCain, who has been a good friend of Powell for decades. Powell, a Republican, has advised McCain in the past on foreign policy. Powell, who served under President George W Bush, told reporters after the interview he had been disturbed in recent weeks by the negative tone of McCain's campaign, particularly its focus on Obama's passing relationship with William Ayers, a 1960s radical and founder of the Weather Underground. The McCain campaign has sought to promote the idea that Obama is "palling around with terrorists". "I thought that was over the top," Powell told reporters. "It was beyond just good political fighting back and forth." In offering his endorsement, Powell becomes the highest profile Republican to add his support to the Democratic ticket. Aides said it was not yet known whether the two men would campaign together - or what Powell would do alone - in the final two weeks of the presidential campaign. Powell met with both McCain and Obama in June in preparation to make a possible endorsement. He has said repeatedly in recent months that he wanted to wait until after the political conventions and the presidential debates before making a decision. Powell's support of Obama was not a surprise to people who know him well and within Washington's foreign policy establishment, but the Obama campaign welcomed it as a powerful reassurance to voters about Obama's national security credentials. Other voters, however, could discount it as an action of a disgruntled member of the Bush administration or as simply the support of one African American for another. Powell also told reporters on Sunday that he was troubled that a number of Americans believe that Obama is a Muslim, although he did not directly link that supposition to the McCain campaign. At a recent town-hall style meeting, McCain told an audience member who said she thought that Obama was an "Arab", "no, ma'am, he's a decent family man." "These are the kinds of images going out on Al Jazeera that are killing us around the world," Powell said. "And we have got to say to the world it doesn't make any difference who you are and what you are. If you're an American you're an American." McCain was asked about Powell's endorsement during an interview on Fox News Sunday. "I have always admired and respected General Powell," McCain said. "We have a respectful disagreement." Obama called General Powell at 10 am to thank him for the endorsement and told him "how honoured he was to have it," said Robert Gibbs, a senior adviser to Obama. The two spoke about 10 minutes. "He said he looked forward to taking advantage of his advice in the next two weeks and hopefully over the next four years," Gibbs said.