The Washington Post's story cited unnamed USA intelligence officials as saying newly-analysed information confirmed that on 23 May senior members of the UAE government had discussed a plan to hack Qatari state media sites.
The four states - the UAE, Saudi Arabia, Egypt and Bahrain - imposed sanctions on Qatar on 5 June, cutting diplomatic and transport ties with the tiny Gulf monarchy, after accusing it of financing militant groups and allying with their regional arch-foe Iran.
Qatar said those reports were fabricated and posted by hackers, though it hasn't identified the source.
Qatar says it believes the Washington Post report, saying it "unequivocally proves that this hacking crime took place".
The officials said it was unclear if the UAE hacked the websites or paid for them to be carried out, the newspaper reported.
Anwar Gargash denied as false a story in the Washington Post that cited USA officials saying the UAE had orchestrated the hack of Qatar's state news agency. Qatar has denied the accusations.
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He pointed out that "the current crisis of dimensions beyond the domestic arena of the GCC (Gulf co-operation council)", noting the "need to put an end to official support for extremism and jihadism and terrorism in various parts of the Arab world".
In late June, Kuwait, as a mediator in the crisis, handed over to Qatar the ultimatum of the four Arab states with 13 demands, including the requests to severe Doha's relations with Tehran, close Turkey's military base in Qatar and shut down the Al Jazeera TV channel, as well as to end support for the Muslim Brotherhood, a terrorist organization banned in Russian Federation.
The Federal Bureau of Investigation was previously known to be working with Qatar to probe the hacking.
Fellow Gulf states had already rejected Qatar's explanation and said Sheikh Tamim's reported remarks reflected deliberate ambiguities in Qatar's policies that have undermined stability in the region. But it has denied aiding jihadist groups linked to al-Qaeda or so-called Islamic State (IS). His visit had yielded little except for a bilateral agreement between the U.S. and Qatar to fight "terrorism".
"What we really do want is we either reach an agreement and Qatar's behaviour changes, or Qatar makes it own bed and they can move on and we can move with a new relationship".
Mr Gargash said Qatar's neighbours were prepared to continue the boycott for months if it did not comply with the list of demands it was handed last month and agreed to worldwide monitoring.
But, he added, the four states would not escalate the boycott by asking companies to choose between doing business with them or with Qatar.