Qatar's former deputy prime minister has accused the United Arab Emirates of plotting to invade Doha with an army of mercenaries.
Abdullah bin Hamad al-Attiyah made the charges on Wednesday in a report published by Spanish daily ABC.
The UAE, Saudi Arabia, Bahrain and Egypt began a blockade on Qatar in June, accusing it of supporting "extremism" and being too close to Iran - changes Doha has denied.
The bloc has shut down air, maritime and land links with Qatar, and imposed economic sanctions.
Attiyah said that the UAE hired a "Blackwater-linked" private security contractor to train thousands of mercenaries to invade Qatar with the aim of overthrowing the emir and replacing him with a ruler subservient to the Saudi-led bloc boycotting the gas-rich state.
The plan, which was prepared ahead of the diplomatic spat, was never carried out because US President Donald Trump failed to greenlight the assault, according to the former senior official.
An unnamed official source told the daily that the soldiers for hire were trained at Emirati military base in Liwa in the west of the country by ACADEMI - a US security service company formerly known as Blackwater.
"We estimate that Blackwater trained about 15,000 employees, most of them Colombian and South American," the source said.
Blackwater military contractors killed 17 unarmed Iraqi civilians and injured 20 in a notorious 2007 massacre in Baghdad that prompted the firm to change its name.
Reports first emerged in 2015 that the UAE was sending mercenaries to fight in Yemen, choosing not to send its own citizens to take part the war against the Houthi rebels.
The country has been accused of transferring running secret prisons in Yemen where torture was said to be widely used against detainees.
"The UAE has not only utilised private security contractors to bolster its own ability for self-defence but has used them to engage in foreign wars and, potentially, domestic repression," said security expert David Isenberg.
According to a recent email purportedly sent by the Emirati ambassador to the US, Yousef al-Otaiba, Saudi Arabia came close to "conquering" Qatar before the start of the blockade.
Sources close to Trump said in September that the Saudi-led bloc considered taking military action against Qatar at the start of the crisis before the US President urged for calm.
Kuwait's emir, who has been leading efforts to mediate an end to the spat, has said he stopped the bloc from taking military action against Qatar.