The brother of Afghan President Hamid Karzai is believed to be linked to drug trafficking, officials told the New York Times.

The newspaper cites two incidents — one in 2004 and the other in 2006 — in which Wali Karzai allegedly pressured Afghan authorities to release confiscated heroin.

An informant who reportedly tipped off U.S. and Afghan authorities about the 2006 drug shipment has been in an Afghan jail since 2007 for allegedly plotting to kill a political rival, the newspaper reported on Sunday.

Wali Karzai has denied any involvement in the heroin trade, according to the New York Times.

"I am not a drug dealer, I never was and I never will be," he told the newspaper recently. "I am a victim of vicious politics."

But U.S. officials are worried that the perception that the Afghan president is protecting his brother could undermine the Afghan government’s authority, the newspaper reported.

Both U.S. and British intelligence officials briefed Hamid Karzai about the allegations in 2006 in the hope that the president would remove his brother from Afghanistan, but the president demanded evidence to support the claims against his brother, officials told the newspaper.

"We don’t have the kind of hard, direct evidence that you could take to get a criminal indictment," a White House official told the New York Times. "That allows [President] Karzai to say, ‘where’s your proof?’ "