European nations worked Monday to keep diplomatic efforts to curb the Israel-Iran war alive as the two countries traded strikes following the United States’ weekend attack on Iran’s nuclear program. Calls for Tehran to enter talks with Washington appeared to fall on deaf ears as it reached out to ally Russia for support.

The move, the first time the U.S. has attacked Iran, is a major escalation of the conflict between Israel and Iran.

F.B.I. and D.H.S. officials cautioned state and local law enforcement of the possibility of threats to U.S. communities.