TOKYO, July 31 (Reuters) - Oil prices gained for a fourth straight day on Thursday, as investors worried about supply shortages amid U.S. President Donald Trump's push for a swift resolution to the war in Ukraine and threats of tariffs on countries buying Russian oil.

Brent crude futures for September delivery , which are set to expire on Thursday, rose 27 cents, or 0.4%, to $73.51 a barrel by 0028 GMT, while U.S. West Texas Intermediate crude for September gained 37 cents, or 0.5%, to $70.37 a barrel.