MOSCOW (Reuters) -The U.S. embassy in Russia warned that "extremists" had imminent plans for an attack in Moscow, hours after Russian security services said they had foiled a planned shooting at a synagogue by a cell from the Afghan arm of Islamic State. The embassy, which has repeatedly urged all U.S. citizens to leave Russia immediately, gave no further details about the nature of the threat, but said people should avoid concerts and crowds and be aware of their surroundings.