ARTEMIVSK, Ukraine —Ukrainian President Petro Poroshenko called Wednesday for an international peacekeeping mission in his nation’s war-torn east, a stark admission that his nation can no longer fend off pro-Russian rebels after a major battlefield defeat.

Any international force on the ground would harden the battle lines after 10 months of fighting, forcing Ukraine to give up for now its attempts to reunify the nation. But it would also halt Russian-backed rebels from pushing onward toward Kiev.

The suggestion came hours after thousands of Ukrainian troops fled the encircled railway hub of Debaltseve, where fighting only intensified after a cease-fire ostensibly took effect Sunday. Nearly a year after Russia annexed the Crimean Peninsula, the fresh loss threatened tough political consequences for Ukraine’s pro-Western president amid questions of how the troops became surrounded in recent weeks.

Soldiers described a chaotic nighttime retreat over eastern Ukraine’s frozen steppe, with shells raining down on them from two sides.

The prospects for a peacekeeping mission in Ukraine were not immediately clear. Any U.N. Security Council mandate would be subject to a possible Russian veto. Poroshenko said he hoped for a European Union police mission, although what such a plan would entail on the ground remained unclear. Any EU-only plan appeared likely to be rejected by Russia, which has said that it views NATO’s encroachment on its borders as a security threat.

Poroshenko has staked his office on reuniting Ukraine and quelling Europe’s bloodiest conflict since the Balkan wars in the 1990s.

The violence may increase pressure on President Obama to supply Ukraine’s military with weapons, a decision he said would be made only after the peace effort. EU leaders, meanwhile, said they would consider more economic sanctions against Russia.

Elsewhere in Ukraine’s war-torn east, violence was abating as rebels announced that they had begun pulling back heavy weaponry in accordance with the cease-fire agreement. But the advance on Debaltseve suggested that the Russian-backed rebels had the strength to push forward when they wished.

Poroshenko called the retreat a “planned and organized withdrawal of certain units from Debaltseve.” During a late-night meeting with top security advisers, the president invited discussion of “an invitation to a U.N. peacekeeping mission,” according to Ukrainian news outlets.

The defeat was sure to stir a political cauldron over the prosecution of the war in Kiev.