A deadly attack on a tanker in the Persian Gulf pushed oil prices to their highest levels in weeks on Thursday, with the international benchmark for futures rising more than 5% earlier in the day before a post from President Donald Trump helped pare some of those gains.

Brent crude futures remained elevated at $103.80 by midday. That still puts them on track for their highest close since late September. But Trump's comments that the U.S. would "not be attacking Iran at any time prior to the Midterm Elections" on Nov. 3 seemed to have calmed investors.

Elevated crude prices reflect mounting concerns about dwindling global reserves as demand from China starts to rebound.

"Multiple projectiles" struck a tanker located off the northwest coast of Qatar, around 10 p.m. local time last night, according to the United Kingdom Maritime Trade Operations, which also reported casualties.

While such attacks can slow or even temporarily halt some transits from the gulf, the bigger problem is the perilously low global oil supply.

Global oil reserves are at 6 billion barrels, down from 10 billion barrels at the beginning of the war, Saudi Aramco CEO Amin Nasser said at this week's Energy Intelligence Forum in London. That includes the U.S. strategic reserve of 283 million barrels, its lowest level since 1982, according to Energy Department data.

"The supply resilience cushion is scarily thin," Nasser said. That can have much broader repercussions, including slower economic growth and higher inflation, he added.

"The longer the disruption continues, the risk of this happening only grows," he added.

Meanwhile, lower demand for oil from China, which sharply cut back imports as the Iran war broke out, is "likely mostly unsustainable," analysts at Goldman Sachs wrote in a Thursday morning note.

China's curtailed imports have been a key factor in keeping a lid on oil prices while the Strait of Hormuz was effectively closed early in the conflict. But its demand for oil products to produce industrial petrochemicals such as ethylene and propylene used to make everything from auto parts to plastic bottles had already recovered by some 30% in August, the analysts wrote.

While switching to different fuel sources -- including electricity for vehicles -- has helped lower China's need for gas and diesel, most of the muted demand for the oil used in its petrochemical industry was driven simply by reducing production of those chemicals. But that can only go on for so long.

"China is unlikely to continue to draw down its chemical inventories for multiple years for reasons similar to why they seek to keep their crude oil inventories elevated -- to attain self-sufficiency and hedge against future supply shocks," the analysts wrote.

All that puts even more pressure on global oil reserves.