The vessels — the Kairos, a 274-meter tanker, and the Virat, an aframax-class ship — were traveling in waters north of Turkey when each reported what officials described as an external impact. Both ships are among the group of vessels targeted by Western sanctions for transporting Russian crude. In recent years, these tankers have typically operated with limited transparency, patchy regulatory oversight and irregular insurance arrangements, characteristics that have unnerved risk assessors even in quieter weeks.