Falling oil prices are adding to the downdraft in stocks, as the decline in Canadian and U.S. markets gained speed Tuesday.

U.S. stocks slumped back into negative territory for 2018 and Canadian stocks were hit again by tumbling oil prices, as investors weigh further evidence that global economic activity is slowing.

The Standard and Poor’s 500, which declined 1.8 per cent, is now down 10 per cent since hitting its high point in September, while Canada’s S&P/TSX Composite Index closed at its lowest level in two years, weighed down by Tuesday’s 6.6-per-cent decline in crude oil.

West Texas Intermediate oil, a U.S. benchmark for crude, fell to US$53.43 a barrel, touching its lowest level in a year.

Oil’s downward slide, part of a 30-per-cent retreat since early October, comes as observers watch for slowing demand for energy amid signs of a cooling global economy. But Tuesday’s drop may have also had to do with geopolitics: the decline accelerated after U.S. President Donald Trump said he stood by Saudi Arabia, despite reports that the Central Intelligence Agency has concluded that Saudi Crown Prince Mohammed bin Salman likely ordered the killing of journalist Jamal Khashoggi last month.

Mr. Trump’s comments were seen as supportive of weak oil prices, given that Saudi Arabia meets with the Organization of Petroleum Exporting Countries next month to discuss potential production cuts – an action Mr. Trump does not favour.

Canada’s energy-heavy S&P/TSX composite fell 194.01 points, or 1.3 per cent, closing at 14,877.00.

The S&P 500′s decline to 2,641.89 means the index is now down 1.2 per cent for the year. The 30-member Dow Jones Industrial Average fell 551.80 points on Tuesday, or 2.2 per cent, closing at 24,465.64.