LONDON—Turkey’s president brushed off a U.S. plea for restraint in the country’s military offensive against Kurdish fighters in northwestern Syria, a sign of the heightened tension between the two treaty allies.

​Secretary of State Rex Tillerson and U.K. Foreign Secretary Boris Johnson, speaking in London Monday, called on Turkish and Syrian-Kurdish forces in northwestern Syria to exercise restraint and to limit their operations after the start of the Turkish offensive over the weekend.

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