HOUSTON (Reuters) – Exxon Mobil Corp this week banned the use of vessels linked to oil flows from Venezuela in the last year, according to four people familiar with the matter, placing new pressure on the U.S. sanctioned country and on global crude freight rates.

The largest publicly-traded oil producer’s contract clause would affect about 250 tankers, two of the people familiar with the matter estimated. Exxon did not immediately respond to requests for comment. Over 25 foreign vessels touch Venezuelan ports per month, according to PDVSA’s and Refinitiv Eikon data.