Royal Dutch Shell has reported a 15% slump in profits for the last quarter at a time of lower oil prices and a weaker global economy.

The company made $4.8bn (£3.7bn) in the past three months, compared with $5.6bn in the same period last year, and warned it may miss targets to reduce debt and pay back shareholders.

Earlier this week, BP reported a sharp fall in profits for the same period, from $3.8bn a year earlier to $2.3bn.

Oil analysts expect company profits to be weaker across the industry after global prices fell to an average of $62 a barrel in the last quarter, from more than $75 a barrel a year ago, due to worries over the sluggish global economy.

Shell’s chief executive, Ben van Beurden, said weak macroeconomic conditions and a challenging outlook for the global economy and oil market prices would “inevitably create uncertainty” about the pace of Shell’s financial goals.

Shares in Shell fell 3% in early trading in London.

The company had planned to cut its debt to a quarter of earnings and spend $25bn buying back the shares it offered shareholders in lieu of dividends over recent years by the end of 2020.

Jessica Uhl, the chief financial officer of Shell, said she had “complete confidence” that Shell would meet its ambitious financial targets, but that they were always contingent on oil market conditions.

If oil and gas market prices remain sluggish this could set Shell’s plans back by between $8bn to $9bn and delay the 2020 timeline, Uhl said.

Shell had expected oil prices to average $65 a barrel this year, but prices have remained below this level for much of the year due to uncertainty over the world’s future demand for energy. Oil market traders fear a global economic slowdown, triggered by the US-China trade war, will dent demand for oil.

“We don’t know what will happen,” Uhl added. “As we’ve seen in the last year, we’ve seen oil prices start at $55, reach $85 and then trend lower again.”

The uncertainty is a blow to his ambitions to create a “world-class investment case” for the company while strengthening Shell’s “societal licence to operate” by helping tackle climate change.

Shell has made bold promises to become the world’s biggest electricity company in a radical move towards clean energy.

But a Guardian investigation this month revealed Shell also plans to increase its fossil fuel output by 38% by 2030, by raising crude oil production by more than half and gas production by more than a quarter.

The fossil fuel binge is forecast to take place despite efforts to limit global heating to 1.5C by the end of the next decade to prevent runaway climate chaos.

The company has produced on average 3.6m barrels of oil equivalent a day this year, in line with oil and gas production last year.