Saudi Arabia plans to invest $600bn in the United States over the next four years, Crown Prince Mohammed bin Salman told former President Donald Trump in a phone call on January 22, marking Trump's first reported foreign leader contact since returning to office.

"The crown prince affirmed the kingdom's intention to broaden its investments and trade with the United States over the next four years, in the amount of $600bn, and potentially beyond that," the Saudi Press Agency reported.

The Saudi prince’s call comes after Trump in a meeting with the press said that the cost of a visit to the Kingdom would cost the Saudis at least $500bn if he was to make it his official foreign trip.

"The first foreign trip typically has been with the UK but... I did it with Saudi Arabia last time because they agreed to buy $450bn worth of our products," Trump told journalists on January 21.

While the readout did not specify investment targets, Saudi Arabia remains heavily reliant on US defence systems and weapons, which could form part of the package. The kingdom's sovereign wealth funds have also taken significant stakes in American businesses and sports investments.

The crown prince also spoke with US Secretary of State Marco Rubio on January 22.

The conversations mark early diplomatic engagement between Saudi Arabia and the new Trump administration, which follows Trump's historically close ties with the kingdom during his first term.

The massive investment commitment, which exceeds many nations' GDP, comes as Saudi Arabia faces its own budgetary constraints amid depressed oil prices.

The kingdom is also funding Crown Prince Mohammed's $500bn NEOM megaproject and preparations for hosting the 2034 FIFA World Cup, which has become growing black hole of funds including the so-called The Line project which has been significantly cut back in recent months.

Initially, the government aimed to house approximately 1.5mn residents in The Line, a cutting-edge urban area within Neom, by 2030, which would squeeze people into a giant skyscraper initially stretching deep into the desert. 

Neom is envisioned as a vast, eco-friendly urban development initiative, integral to Crown Prince Mohammed bin Salman's Vision 2030 to shift the Saudi economy away from oil dependency. 

The project aims to operate on renewable energy, support sustainable tourism, and incorporate advanced technologies, including robotics, to assist its inhabitants in daily life, with a strong commitment to environmental conservation.