Qatar has accused its Gulf neighbours of breaking international law by hacking government websites and planting false information that helped cause a continuing diplomatic rift in the region.

According to the Washington Post, US officials discovered last week that ministers from the United Arab Emirates held a meeting on 23 May to discuss plans to hack Qatari government news and social media sites and post incendiary false quotes attributed to Qatar’s emir, Sheikh Tamim Bin Hamad al-Thani.

The alleged hack, which involved disparaging remarks purportedly by the emir about Donald Trump, praise for Hamas and support for Iran as an “Islamic power”, took place the following day. It preceded the current split in the Gulf between Qatar and a coalition of four states – the UAE, Saudi Arabia, Egypt and Bahrain – that are maintaining an economic and diplomatic boycott against it.

The group 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 foe Iran. Doha denies the accusations and says Thani never made the remarks.

The Qatar information office said: “The information published in the Washington Post ... revealed the involvement of the United Arab Emirates and senior Emirati officials in the hacking of Qatar News Agency.

“[The report] unequivocally proves that this hacking crime took place.”

Qatar had asked the FBI to investigate the source of the alleged hack, but no official US confirmation emerged on Monday that it believes Qatar’s rivals were responsible.

The UAE foreign minister, Anwar Gargash, in the UK to deliver a lecture on the causes of the Gulf crisis at Chatham House, said on Monday: “The Washington Post story is not true. It is purely wrong. You will see in the next few days the story will die.”

Gargash denied that the alleged hack could have precipitated the crisis, saying “this issue has been festering since 2014”. Asked if he believed that the Qatari emir had expressed the views in question, he said: “If you look at the comments they are very consistent with what they have said.”

It is possible that the US intelligence finding has been leaked by government officials frustrated that the UAE has not shown a greater willingness to compromise.

Gargash implied that the Gulf Cooperation Council may collapse, rather than see other member states throw Qatar out. “The GCC is in crisis and I don’t think it serves our purposes to say ‘let’s take Qatar out’,” he said.

“What we really do want is we either reach an agreement and Qatar’s behaviour changes, or Qatar makes its own bed and they can move on and we can move with a new relationship. But we cannot have a member who is undermining us and supporting extremism.

“You cannot be part of a regional organisation dedicated to strengthening mutual security and furthering mutual interest, and at the same time undermine that security. You cannot be both our friend and a friend of al-Qaida.”

Predicting a possible stalemate in the dispute, Gargash nevertheless claimed that Qatar’s private promise to western powers to review a list of 59 extremists the UAE claims are in Doha is a direct result of the blockade. The UAE wants the individuals arrested or expelled, along with 12 named organisations.

Qatar’s decision last week to sign a memorandum of understanding on terrorist financing with the US was also hailed as “a positive development” by the foreign minister.

Claiming that it has been warning of an extremist threat since the 1980s, Gargash described Qatar as a “very wealthy state, with $300bn (£230bn) in reserves, which is wedded to extremist jihadism and terrorism”.

The tone of his speech is likely to disappoint the succession of western foreign ministers who have travelled to the Gulf to mediate. They have warned that the confrontation is threatening long-term stability and investors may pull out if it is not settled quickly.

“Understandably many of our friends in Europe and beyond are concerned about this crisis,” Gargash said. “They see the Arab Gulf as a haven of stability in an unstable Middle East, and as an important and functioning common market.

“Many would argue that it is one of the few Arab bulwarks against further Iranian expansion. We understand and respect those concerns.

“But as we know from meetings with American and European officials, they are also aware of Qatar’s duplicity.”