ISLAMABAD, Pakistan — Pakistan on Friday placed travel restrictions on United States diplomats based in the country, the latest in a series of retaliatory measures that threaten to plunge already strained relations to their worst level in years.

The restrictions were imposed on the same day that the United States barred diplomats working at the Pakistani Embassy in Washington from traveling outside of a 25-mile radius around the city without approval.

The United States has long complained that Pakistani police and security officials frequently harass American diplomats and their staff with traffic stops and citations that require considerable time and effort to resolve. Six weeks ago, the State Department threatened to impose a travel restriction on Pakistan’s Washington diplomatic corps if the harassment did not end by Friday.

On Friday, American officials imposed the restrictions.

In a letter from Pakistan’s Ministry of Foreign Affairs delivered to the American Embassy in Islamabad, Pakistani officials said that they had established a fast-track mechanism on April 27 to address American complaints about harassment but had yet to receive a single complaint through it.