The incident helped spark a diplomatic rift between Qatar and its neighbours.
The Washington Post story also comes days after the USA secretary of state Rex Tillerson spent four days engaged in shuttle diplomacy between Kuwait, Qatar and Saudi Arabia - all strategic U.S. partners - seeking to find a way to resolve the crisis.
Qatar said in late May that hackers had posted fake remarks by the emir, an explanation rejected by Gulf states.
Swiss news network The Local said a fake news story quoting Federation Internationale de Football Association president Gianni Infantino had been posted on a copycat website on Saturday.
Riyadh, Abu Dhabi and allies in Egypt and Bahrain cut trade and political ties with Qatar on 5 June, claiming that Doha meddled in global affairs and funded terrorist organisations.
"What is true is Qatar's behavior".
It was not clear whether the UAE hacked the Qatari sites itself or paid another entity to do so, The Post added.
Qatar's antagonists accuse it of supporting terrorism and giving a home to extremists inciting violence and radicalisation.
The Federal Bureau of Investigation was previously known to be working with Qatar to probe the hacking.
"The Washington Post story today that we actually hacked the Qataris is also not true", he told the London-based think-tank Chatham House.
US Secretary of State Rex Tillerson on Thursday returned to the United States from his shuttle diplomacy in the Gulf region to try to resolve the dispute. The article stated that the officials were still not certain whether the hacks were carried out by contractors or officials.
Khalil Jahshan, executive director at the Arab Center Washington, DC, told Al Jazeera the revelations are the most important development so far since the beginning of the crisis and undermined the Emirati position.
The report "unequivocally proves that this hacking crime took place", it added.
Last month, Mr Al Otaiba's emails were hacked and released by a group linked to Qatar called GlobalLeaks.
Qatar has been subjected to a diplomatic and economic blockade that the USA government says could compromised U.S. efforts against so-called Islamic State.