In the weeks preceding Wednesday’s Minsk summit talks, pro-Russian rebels, fortified by fresh supplies of Russian weapons, seized the ruins of the Donetsk airport and launched a determined offensive against the town of Debaltseve, an important rail junction and the last pocket of Ukrainian-held territory in a wide swath of territory north of Donetsk.

The gap between Europe’s dogged diplomacy and Mr. Putin’s approach to Ukraine, which mixes regular calls for peace with stealthy supplies of Russian weapons and even soldiers to the separatists, has left Moscow and Brussels “playing entirely different games,” said Amanda Paul, a Russia expert at the European Policy Center, a Brussels research group.

“Putin can outmaneuver us because he knows what our limits are. He knows we will not deploy troops. He knows that even if the United States decides to send some arms, Russia is still strong enough to defeat Ukraine,” she added. “But we have no idea what Putin’s limits are. He does not show his cards. The West does. Maybe there is not a military solution, but we should keep Russia guessing.”

While vague on his objectives in Ukraine beyond a determination to block what he sees as a plot by NATO to push deep into former Soviet territory, Mr. Putin has made clear his desire to divide the European Union, reaching out to countries like Hungary and Greece, where a new left-wing government has raised doubts about the wisdom of sanctions imposed on Moscow over Ukraine. So far, however, the 28-nation bloc has managed to stay unusually united, in part because it has stuck to relatively modest sanctions, in step with Washington.

Linas A. Linkevicius, the foreign minister of Lithuania, one of the few European countries that support sending weapons to Ukraine, said, “Diplomatic efforts are of course worthwhile, but we can’t judge whether they are working, even if we get Russia’s signature on an agreement. We have learned that we can only judge events on the ground.”

“We cannot trust a single word of the Russian leadership,” he added. “They are all worthless.”

The last Minsk agreement began to unravel almost as soon as it was signed. There were violations of the cease-fire on both sides, but particularly from the rebels, whose leader at the time, Pavel Gubarev, denounced the accord as a sellout and declared “we want to spit on this ‘peace.’ ”

Since then, the rebels have consolidated their previously fragmented land into a more defensible territory and have vowed to resist any return to a so-called “demarcation line” fixed in September. Mr. Fabius, the French foreign minister, indicated that France and Germany, which have taken the lead in Europe’s diplomatic push, would accept revisions to the earlier accord, saying that it would be respected “as far as possible.”