Dock workers haul the mooring rope of a cargo ship onto the dockside at Bandar Imam Khomeini (BIK) port on Friday, May 24, 2019.

President Donald Trump is threatening to impose "major" new sanctions against Iran on Monday, ramping up the pressure on the Islamic Republic at a time when its economy is straining under the weight of financial restrictions.

Six oil tankers and a U.S. spy drone have been attacked since May either in or near the Strait of Hormuz — the world's busiest transit lane for seaborne oil shipments that separates Iran from its neighboring Gulf states.

The flurry of attacks has escalated tensions between Washington and Tehran.

On Saturday, Trump said via Twitter that he would impose additional sanctions against Iran in a bid to prevent the country obtaining nuclear weapons. The U.S. already has sanctions on Iran's oil industry and other sectors.

Trump added that economic pressure would be maintained unless Iran's leadership changed course.

"We can safely say that Iran's revenue from oil has been cut by at least two-thirds, so they are in a very dangerous economic position," Cailin Birch, global economist at the Economist Intelligence Unit, told CNBC's "Squawk Box Europe" on Monday.