The White House said Tuesday that Russia and Ukraine had agreed to a ceasefire in the Black Sea and to implement a ban on attacks on energy facilities by the two neighbors, an apparent breakthrough after American negotiators held separate talks with both countries.

If implemented, the agreements could mark a major step toward a wider deal to end the three-year-long war, which began when Russia launched a largescale invasion of its smaller neighbor.

Negotiators had agreed both countries would…