"We are not going to fight the Russians in Ukraine and we have been very clear and open about that. We also have to make a very sharp distinction between the guarantee that we extend to our NATO partners, including the Baltic states, where there is a hard red line protecting them," Hammond said.
The British diplomat was briefly interrupted by the host of The Andrew Marr Show to clarify whether "any Russian incursion" toward the 28-member military bloc would result in war.
Article 5 of the Washington Treaty asserts a unified security doctrine where an attack against one member may be countered by all members with the use of armed force.
Western countries have maintained their accusations that Russia is a party to the internal conflict in Ukraine, claims unequivocally and repeatedly denied by Moscow as groundless.
Russia, for its part, has on numerous occasions pointed to an unprecedented expansion of NATO toward its western border.
