Oil prices jumped to their highest level in a month on Friday as renewed hostilities between the United States and Iran have caused shipping traffic in the Strait of Hormuz to slow to nearly a halt.

The price of Brent crude, the international benchmark, climbed about 4 percent, to about $88 a barrel, the highest since early June but still below its peak in April, when oil prices soared above $120.

West Texas Intermediate crude, the U.S. benchmark, was up 4 percent to $82 a barrel.

Oil prices rose as the Iran war, now in its fifth month, continued to disrupt the supply of energy. Just eight ships navigated the strait on Thursday, the second full day of the reinstated U.S. naval blockade of Iran.

Shipping activity was at its lowest level in more than a month, down from 13 ships the day before, according to Kpler, a maritime data firm. Most of the vessels that navigated through the strait on Wednesday and Thursday passed through the Tehran-mandated corridor in Iranian waters.

The United States carried out more attacks against Iran on Friday as the fighting continued for the seventh straight day.

The U.S. blockade on Iran is likely to severely hamper Tehran’s ability to monetize its control over the strait, as it did during the first blockade, which lasted from April until mid-June. During that period, U.S. forces redirected or disabled more than 149 ships, depriving Iran of billions of dollars in oil revenue.

U.S. Central Command said in a statement that as of Thursday, American forces had redirected three commercial vessels trying to evade the blockade, disabled a ship that did not comply and boarded another ship. The military said the boarding happened on a Cook Islands-flagged tanker, the Wen Yao.

Fuel prices have also continued to go up, with the average price of gasoline in the United States on Friday moving 4 cents higher to $3.98 a gallon and diesel up 5 cents to $5.06 a gallon, according to the AAA motor club.