WASHINGTON — In the early morning of June 6, a uniformed Russian Federal Security Service guard stationed outside the US embassy in Moscow attacked and beat a US diplomat who was trying to enter the compound, according to four US officials who were briefed on the incident.

This previously unreported attack occurred just steps from the entrance to the US Embassy complex, in the Presnensky District in Moscow’s city center. After being tackled by the guard, the diplomat suffered a broken shoulder, among other injuries. He was eventually able to enter the embassy and was then flown out of Russia to receive urgent medical attention, administration officials told the Washington Post. He remains outside of Russia.

The attack caused a diplomatic episode behind the scenes that has not surfaced until now. The State Department in Washington called in Russian Ambassador Sergey I. Kislyak to complain about the incident, an administration official said.

The motive for the attack remains unclear. One US official said the diplomat was seeking refuge in the embassy complex to avoid being detained by the Russian intelligence services. A different US official said the diplomat may have been working as a spy in Russia under what’s known as ‘‘diplomatic cover,’’ which means he was pretending to be a State Department employee.

Washington Post