Qatar has agreed to buy "tremendous amounts of military equipment" and Boeing planes from the United States following a visit by Gulf Nation's Emir to the White House, according to President Donald Trump.

Key points: Deals between Qatar and the US are believed to be worth tens of billions of dollars

Deals between Qatar and the US are believed to be worth tens of billions of dollars Bahrain, Egypt, Saudi Arabia and the UAE have boycotted Qatar since June 2017

Bahrain, Egypt, Saudi Arabia and the UAE have boycotted Qatar since June 2017 The blockading countries have accused Doha of supporting extremist groups in the region

The weapons and commercial deals are tipped to cost tens of billions of dollars, as Mr Trump touted agreements with Qatar — a nation that is regarded as a pariah state among its Arab neighbours.

"They are investing very heavily in our country," Mr Trump said.

"They're buying tremendous amounts of military equipment, including planes.

"And they're buying commercial planes, as you know — very large numbers of commercial planes from Boeing. And we very much appreciate it."

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The deals come at a time when US company Boeing is embroiled in an ongoing crisis — it is poised to be overtaken by rival Airbus as the world's biggest plane maker after two of its planes nosedived in Indonesia and Ethiopia, killing 346 people and causing Boeing stocks to plummet.

At a state dinner Mr Trump warmly welcomed Qatari Emir Sheikh Tamim bin Hamad Al Thani, saying they had been "friends for a long time".

They were joined by several influential figures, including Goldman Sachs executive Dina Powell, International Monetary Fund managing director Christine Lagarde and Robert Kraft, the billionaire owner of the New England Patriots football team who earlier this year pleaded not guilty to prostitution charges.

But the meeting and trade deals come amid a bitter rift between the tiny, energy-rich nation and its fellow American allies in the Persian Gulf, who claim Qatar supports terrorist groups in the region.

Mr Trump is facing critical decisions in the Middle East, and the latest development comes as tensions heighten between the US and Iran.

Meanwhile, there is an ongoing blockade imposed on Qatar by its Arab neighbours.

Why is there a boycott against Qatar in the Middle East?

Four Arab nations severed ties with Qatar's Emir Sheikh Tamim bin Hamad Al-Thani's regime in 2017. ( AP: Osama Faisal )

Four Arab nations with strong diplomatic relations with the US — Bahrain, Egypt, Saudi Arabia and the UAE, with support from government bodies in Libya and Yemen — have long-boycotted Qatar claiming it supported extremist and terrorist groups in the region.

The countries severed their diplomatic ties with Qatar in June 2017, and they stopped Qatar Airways flights from using their airspace, closed off the small country's sole land border with Saudi Arabia to drain the country's supplies, and blocked its ships from using their ports.

The blockading countries have accused Doha of having strong ties with Iran, interfering in regional affairs and supporting "terrorism". Qatar has strongly denied these allegations.

The backdrop to this Middle East powerplay is a fierce rivalry between Saudi Arabia and Qatar for influence in the region, as well as Mr Trump's increasingly close relationship with the Saudis.

The rivalry was exacerbated following the 2011 Arab Spring when a regional narrative war ensued between Arab state media outlets (who backed the region's long-serving dictators) and Qatar's Al Jazeera, (who backed the overthrowing of those leaders).

As protest after protest was squashed from Egypt, to Bahrain, to Syria, while Al Jazeera was banned, the US-Saudi friendship served to embolden the Saudis and the UAE to put Qatar back in its place, according to James Piscatori, deputy director at ANU's Centre for Arab and Islamic Studies, who previously told the ABC the blockade would have been unlikely without "the intervening variable of Trump".

Why did Trump ink deals and what do they include?

The price tags on the other deals were not disclosed, but those familiar with the transactions said they totalled tens of billions of dollars. ( AP: Evan Vucci )

Despite the regional economic and diplomatic stand off, Qatar maintains strategic military importance for the US.

A forward headquarters of the US military's Central Command is stationed in Qatar, and the sprawling Al Udeid Air Base is home to about 10,000 American troops.

It's the largest US military installation in the Middle East, and the air base "has served as the primary staging ground for most air operations in the campaign to defeat ISIS," according to the US Department of Defence.

In the past, Mr Trump has also accused Qatar of being a "funder of terror", but Treasury Secretary Steven Mnuchin this week praised the emir for his help in combatting terrorist financing.

The White House also said the Qatari defence ministry would buy a missile defence system from military contractor Raytheon.

Qatar Airways is buying five Boeing 777 Freighters and large-cabin aircraft from Gulfstream Aerospace and plans to use General Electric jet engines to power its Boeing aircraft.

Additionally, Chevron Phillips Chemical and Qatar Petroleum have agreed to jointly develop a $US8 billion ($11.54 billion) petrochemical plant on the US Gulf Coast.

The price tags on the other deals were not disclosed, but those familiar with the transactions said they totalled tens of billions of dollars.

The national airline was one of several businesses that inked agreements for business with US companies on defence, energy and aviation projects.

Speaking at a dinner Monday night (local time), the emir referred to the US as "partners, allies and friends".

"We continue our mutual commitment to remain closely invested in both military and security alliance," he said.

"Unfortunately, there are some in my region who do not share our beliefs."

Qatar, which has one of the world's highest per capita incomes due to its natural gas reserves, has worked to open up other avenues of income to offset the impact of the boycott.

"We have a lot of investments in the US," the emir said.

"We trust the economy here. We do a lot on infrastructure and we're planning to do more investment."

ABC/wires