The evidence has been mounting for some time, but there is no longer any doubt: Russian troops are in Ukraine, not as volunteers, as the rebel commander in Donetsk would have the world believe, but in units equipped with mobile artillery and heavy military equipment.

A senior NATO officer reported on Thursday that the alliance had monitored a “significant escalation” in Russia’s military “interference” in Ukraine, and that well over 1,000 Russian soldiers are operating inside the southeast of the country. The officer, Brig. Gen. Nico Tak, called it “interference,” presumably because “invasion” would mean an all-out military assault. But, by any name, it is a large and unacceptable escalation of Russia’s aggression against Ukraine.

President Obama was right in his news conference on Thursday to rule out military action, but new, tougher Western economic sanctions are obviously needed to make clear to President Vladimir Putin of Russia that the West views his lies and escalating aggression as a major threat.

Mr. Obama declined to speak of an “invasion” in his relatively restrained comments, describing the latest developments only as “a continuation” of what Russia has been up to and of his “expectation” that the allies will deepen the sanctions. His ambassador to the United Nations, Samantha Power, was more blunt in her address Thursday to the Security Council, where she described Russia’s actions as “a deliberate effort to support, and now fight alongside, illegal separatists in another sovereign country.” That should be the reality guiding Mr. Obama when he meets with NATO allies in Wales next week.