Washington streets have buses running around endorsing advertisements that link "Islamic Jew-hatred" Islam with Adolf Hitler, with U.S. capital's mass transit authority claiming it is legally powerless to ban them.

Featuring a photo of the Nazi German dictator in conversation with "his staunch ally" Haj Amin al-Husseini, grand mufti of Jerusalem during World War II, 20 Metro buses are sporting the elongated broadsides, Agence France-Presse reported.

"Islamic Jew-hatred: It's in the Quran. Two-thirds of all U.S. aid goes to Islamic countries. Stop racism. End all aid to Islamic countries," the ad states, over a fine-print disclaimer from the Metro transit authority.

Sponsored by the American Freedom Defense Initiative (AFDI), the ads aim to "raise awareness of the depredations of Islamic supremacism," according to its website.

The campaign, which is due to run until mid-June, has been condemned by the Council on American-Islamic Relations (CAIR).

With the help of an online crowd-funding campaign, the campaign hopes to raise $20,000 by Friday. About $7,500 had been yielded as of Tuesday.

"We're not able to refuse ads on the basis of content," a spokeswoman for Metro told AFP, citing a 2012 court case that allowed another AFDI bus ad on the grounds that it was free speech protected by the U.S. Constitution.

On its website, AFDI co-founder Pamela Geller called the campaign a direct response to like-sized Washington bus ads placed in April by American Muslims for Palestine (AMP) which read: "Stop U.S. aid to Israel's occupation," according to AFP.

"So many folks are unaware of Islamic history and the role of Muslim world during the Holocaust... Let's buy more ads," blogged Geller on her eponymous website that included a link to the Indiegogo fundraising page.

Such "inflammatory" ads were clearly intended "to promote hatred of Islam and Muslims" while also whipping up publicity to raise funds for AFDI, CAIR spokesman Ibrahim Hooper said.

He told AFP that CAIR, a Muslim civil rights group, is developing on its own bus ads "to promote mutual understanding as a response to Geller's hate ad." In the meantime, he added, it's giving away free Korans.