Barack Obama's Muslim outreach adviser has resigned from the US Democratic presidential candidate's campaign after reports emerged about a previous link to an imam with ties to Egypt's Muslim Brotherhood.

Mazen Asbahi, a Chicago lawyer, stepped down from Obama's campaign on Monday, according to a report in the Wall Street Journal newspaper published on Wednesday.

"I am stepping down from the volunteer role I recently agreed to take on with the Obama campaign as Arab-American and Muslim-American co-ordinator in order to avoid distracting from Barack Obama's message of change," Asbahi said in a statement the paper reported.

Asbahi sat briefly on the board of Allied Assets Advisors Fund in 2000 alongside Jamal Said, an imam at a mosque in the state of Illinois who the paper said had been linked to the Muslim Brotherhood, an Islamist political group in Egypt.

Said had also been investigated by US authorities over fundraising for Hamas, a Palestinian political faction, the newspaper reported.

Muslims angry

US Muslim groups said the resignation was the result of a campaign to discredit Muslims' involvement in politics.

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"They use smear tactics to attack Muslims who want to take part in the political process," he said.

Hooper said that Muslims were becoming better organised politically and that that may be the reason for the increase in attacks on them.

Obama had apologised to two Muslim women wearing headscarves who were prevented from sitting behind his podium by campaign volunteers during a speech in Detroit in June to stop them appearing in photographs of the event.

Salam Al-Marayati, the executive director of the Muslim Public Affairs Council, where Asbahi had previously worked, told Al Jazeera that the resignation was symptomatic of a deeper problem in US political culture.

"People have to be exposed to the truth.  They don't believe us when we talk about Islamophobia.  They say we're whining or into self-victimisation.  Things like this prove that's not true."