MOSCOW (AP) — Three buses believed to be carrying expelled diplomats have departed from the U.S. Embassy in Moscow.

Before the Thursday morning departure, journalists outside the embassy compound saw people leaving the residences, placing luggage on trucks. Some toted pet-carriers.

Russia last week ordered 60 American diplomats to leave the country by Thursday, in retaliation for the United States expelling the same number of Russians.

The moves were part of a deepening dispute over the nerve-agent poisoning in Britain of a Russian former double-agent and his daughter. Britain alleges Russian involvement, which Moscow vehemently denies.

More than 150 diplomats have been expelled by Britain and allies, and Russia has ordered reciprocal moves.