MOSCOW — A week into a fragile cease-fire between government troops and pro-Russian rebels in Ukraine, that country’s prime minister offered a pessimistic assessment of its chances of holding and said President Vladimir V. Putin of Russia intends eventually to conquer “all of Ukraine.”

Only the military backing of the North Atlantic Treaty Organization can defend Ukraine, the prime minister, Arseniy P. Yatsenyuk, told a conference of security experts in Kiev on Saturday, according to Ukrainian news reports.

Mr. Yatsenyuk reiterated his support, voiced in the summer, for the unlikely eventuality of Ukraine’s joining NATO. This was a shift from his position, and that of others in the pro-Western coalition that came to power after street protests last winter, and illustrates a hardening of anti-Russian sentiment in the Ukrainian elite after months of war.