Russian soldiers, tanks and heavy artillery began rolling into southeastern Ukraine in earnest Thursday, the Ukrainian government said, as well-armed detachments captured key towns, burned buildings and sent the underequipped Ukrainian forces into full retreat — a show of military force that the United States now considers an invasion in all but name.

U.S. officials began saying privately for the first time Thursday that they consider the escalation of recent days tantamount to a Russian invasion, but President Obama stopped short of using the term at a news conference late in the afternoon. He said the United States will continue to rely on sanctions in an effort to deter Russia.

“Russia is responsible for the violence in eastern Ukraine,” he said. “Russia has deliberately and repeatedly violated the sovereignty and territorial integrity of Ukraine.”

But he said the problem will not be solved with U.S. or outside military action. “A military solution to this problem is not going to be forthcoming,” he said.

Buttressing the Ukrainian accounts, NATO released satellite images Thursday of what it said were Russian artillery, vehicles and troops in and around eastern Ukraine. One image showed what NATO officials said was a convoy with self-propelled artillery in the area of Krasnodon, inside territory controlled by Russian-backed separatists, on Aug. 21.

A satellite image from Aug. 21 shows what NATO says are Russian self-propelled artillery units at an undisclosed location inside Ukraine. (NATO/DigitalGlobe/EPA)

“There is no doubt that this is not a homegrown, indigenous uprising in eastern Ukraine. The separatists are backed, trained, armed, financed by Russia,” Obama said.

Russian actions will be a main topic for the summit of NATO leaders next week in Wales, Obama said.

The Russian Defense Ministry reiterated its position again Thursday, saying no Russian military units had taken part in action in Ukraine, according to the Russian news agency Interfax. Maj. Gen. Igor Konashenkov said that lists of Russian military units circulating on the Internet are fake. Russia has previously admitted that 10 of its paratroopers were captured in Ukraine, but said they were there because they wandered into the country by mistake.

Russian and American diplomats clashed during an emergency meeting of the U.N. Security Council on Thursday, with Samantha Power, the U.S. ambassador to the United Nations, saying that Russia has “outright lied” about its involvement in the conflict.

She said that Russian President Vladimir Putin had spoken of the need to “end the bloodshed as soon as possible” in a meeting with Ukrainian President Petro Poroshenko just Tuesday. At the same time, she noted — citing the NATO satellite imagery — Russian combat units were rolling into the Ukrainian city of Donetsk.

Russian Ambassador Vitaly Churkin countered that Washington should stop interfering and called the Russian soldiers in Ukraine “volunteers.”

“There are Russian volunteers in eastern parts of Ukraine. No one is hiding that,” he said. He suggested that Ukraine was supported by Western advisers and funding.

In Washington, State Department spokeswoman Jen Psaki would not publicly brand Russian actions as an invasion, although other U.S. officials said privately that is the conclusion the United States has made.

“Our focus is more on what Russia is doing, what we’re going to do about it, than what we’re calling it,” Psaki said. “What they’re doing is an incursion. It’s a violation of Ukraine’s sovereignty.”

Russia, she said, has “stepped up its presence in eastern Ukraine,” intervened “directly with combat forces, armored vehicles, artillery, and surface-to-air systems,” and is “actively fighting Ukrainian forces as well as playing a direct supporting role to the separatists proxies and mercenaries.”

Another U.S. official, speaking on the condition of anonymity to describe the behind-the-scenes diplomacy, said the purpose of Russia’s “armed intervention” may be to try to open a land route to Crimea, which Russia annexed from Ukraine earlier this year. It may also be to test Ukrainian and Western responses in preparation for such a land grab later, the official and Western diplomats said.

In a statement posted on the Kremlin’s Web site early Friday, Putin urged the separatists in the east “to open a humanitarian corridor to Ukrainian soldiers that are surrounded, in order to avoid pointless victims and provide them the an opportunity to freely withdraw from the area of operations.”

In Kiev on Thursday, a grim-faced Poroshenko stood in the rain at the airport and addressed the nation, saying he had canceled his working visit to Turkey after “sharp aggravation” of the situation in the east, “as Russian troops were brought into Ukraine.” He remained closeted with his national security council for much of the day.

Around Ukraine, locals reacted in varying degrees of disbelief and shock. In the capital, Kiev, where pro-European protesters unseated the president in February, which led to the separatist uprising, residents grouped around television sets in cafes to see the latest news from the front. News filtered out that the country would be stepping up its military draft.

In Mariupol, a southern port city not far from the new fighting, a sense of normal life prevailed, to a degree, one businessman said. But hundreds of protesters gathered in the sunlit evening to call for peace.

The situation continued to be grim in the rebel-controlled strongholds of Luhansk and Donetsk, where 11 civilians were killed within the past day. More than 2,200 people have died in the past five months in a conflict that has left more than 35,000 in temporary camps, with other residents going without food and sufficient water. Ukrainians are worried that Russia could cut off gas supplies to the country as the winter months approach.

The Ukrainian military said that about 12:30 p.m. Thursday, two Russian columns of tanks and armored fighting vehicles entered the town of Novoazovsk on the Sea of Azov after firing on Ukrainian army positions with rockets launched from Russian territory, according to a Ukrainian military spokesman. The spokesman, Col. Andriy Lysenko, said that after a pitched battle the overmatched Ukrainian military forces retreated about 20 miles away to a position near Mariupol.

By late in the day, Russians and their separatist counterparts were in complete control of Novoazovsk, the spokesman said, a border town that had been the focus of intense shelling this week.

“There are no Ukrainian solders left in Novoazovsk,” Lysenko said. He estimated, along with NATO, that more than 1,000 Russian soldiers are now in the country illegally.

There have been reports of a Russian BM-27 Uragan missile system in the area, Lysenko said, aggravating a situation that has become “more complicated” in the last 24 hours.

Karoun Demirjian in Kostroma, Russia, and Karen DeYoung and William Branigin in Washington contributed to this report.