nuclear weapons to break the stalemate. the use of nuclear weapons on the battlefield is part of russian military doctrine, and the horrors it could unleash risk involving the u.s. and nato more deeply in this conflict. >> these are smaller yield, smaller weapons, but they still carry lots of radioactivity and they're still a big step along the scale of escalation to strategic nuclear war. >> reporter: ukrainian and russian officials did hold a fifth straight day of ceasefire talks. russia's lead negotiator tonight claiming the two sides have moved much closer on the question of ukraine giving up any ambitions to join nato, but ukraine saying it has agreed to nothing and wants an immediate cease-fire, russian troop withdrawal, and international security guarantees. as the talks drag on, the suffering only gets worse. in the besieged city of mariupol, it's still not clear how many casualties there may have been after a russian