WASHINGTON — President Obama acknowledged Monday that he is considering arming Ukraine’s military if a diplomatic push fails to yield a cease-fire with Russia-backed separatists, but worried that such an approach could do more harm that good.

In a news conference with German Chancellor Angela Merkel, Obama sounded skeptical about the unintended consequences of sending weapons to an army that is so overpowered by Russian forces and raised doubts about whether it might serve only to escalate a war that has already killed more than 5,000 people.

“Can we be certain that any lethal aid that we provide Ukraine is used properly, doesn’t fall into the wrong hands, does not lead to over-aggressive actions that can’t be sustained by the Ukrainians, what kinds of reactions does it prompt, not simply from the separatists, but from the Russians – those are all issues that have to be considered,” Obama told reporters.

“The measure by which I make these decisions is, ‘Is it more likely to be effective than not?'”

The president said no decision had been made and stressed that, for now, the West is united behind another round of negotiations and a regimen of economic sanctions. The leaders of France, Germany, Russia and Ukraine are slated to meet Wednesday in Belarus for more talks.

Despite the professed unity, the question of arming Ukraine has revealed increasing divisions among Western allies. Some key officials, including Obama’s pick to be the next defense secretary, have said they are open to sending arms to help Ukraine push back against the separatists. Merkel, however, has been unequivocal in her opposition to such a plan.

The message is unlikely to force Putin to reconsider his strategy. Despite a steadily ramped-up regimen of economic sanctions that have helped to weaken his country’s economy, the Russian leader has for months rejected calls to withdraw support for separatist groups fighting to join Russia in eastern Ukraine.

The leaders vowed Monday that they would not “stand idle and simply allow the borders of Europe to be redrawn at the barrel of the gun,” Obama said.

Obama and Merkel met privately Monday for a series of meetings initally aimed at sketching out an agenda for the Group of Seven summit of world leaders in German in June. But the escalating crisis in Ukraine dominated the morning talks, Obama said.

Also on the agenda Monday was the looming deadline in the Iran nuclear talks. Obama told reporters he saw the end of March deadline as a final decision point for the Iranians. After two extensions and months of negotiations, “the issues now are sufficiently narrowed and sufficiently clarified,” Obama said.