KIEV — Russia has stepped up its direct involvement in fighting between the Ukrainian military and separatist insurgents, unleashing artillery attacks from Russian territory and massing heavy weapons along the border, Ukrainian and US officials say.
Russia’s aim, the officials say, appeared to be to stem and perhaps roll back gains made by government forces, who have been retaking rebel-held territory and trying to seal the border. They say Russia’s accelerated intervention raises the prospect of more direct and more heated fighting between Ukraine and Russia.
US officials, citing military intelligence, including satellite images, warned that Russia appeared to be preparing to arm the rebels with more high-powered weaponry than it has previously supplied, including tanks, armored vehicles, and powerful Tornado multiple rocket launchers.
Among the gains of the Ukrainian forces, after days of intense fighting, was the recapture of Lysychansk. The city had been a rebel stronghold, posing a strategic obstacle to government troops pressing through the Luhansk region from the north and west.
The Ukrainian military called in air support as its ground troops struggled to expel insurgents from the city of more than 100,000 people, on the right bank of the Seversky Donets River.
The reports of increased hostilities suggest that, rather than scale back its military intervention in Ukraine after the Malaysia Airlines jet was shot down by a missile that the United States and Ukraine say came from Russia, Moscow is instead devoting even more firepower to the pro-Russian separatists’ cause, in the face of international condemnation and toughened US sanctions.
A NATO military officer, who spoke on the condition of anonymity because the comments were about sensitive intelligence matters, said by telephone, “The United States has shared intelligence information with NATO today regarding strikes that are occurring from within Russian territory, firing into Ukraine territory.”
In Kiev, the Ukrainian capital, a military spokesman said Friday that Ukrainian troops were coming under increased fire from the Russian side of the border, and that the Ukrainian military had recently shot down three Russian surveillance drones.
One was used to target a Ukrainian base near the town of Amvrosiivka, which then quickly came under heavy rocket attack, the spokesman said.
The military spokesman, Andriy Lysenko, said Ukrainian forces were engaged in particularly heavy fighting near a border crossing at Chervona Zorya, not far from where two Ukrainian fighter jets were downed Wednesday in what Ukrainian officials said was a missile attack from the Russian side of the border.
“We have facts of shelling of Ukrainian positions from the territory of Russian Federation,” Lysenko said. “We have facts on the violation of air border between Ukraine and Russia.”
Lysenko said some Russian soldiers had surrendered to Ukrainian forces. “We have information about weapons and mercenaries, who have respective skills for warfare, who have been passing over from the territory of Russian Federation,” he said.
Russia has repeatedly denied that its forces are involved in the fighting in eastern Ukraine and that it is supplying rebels with weapons and other equipment, despite a substantial body of evidence collected by Ukraine and its allies.
Ukraine and the United States have also said the missile that destroyed Malaysia Airlines Flight 17 was fired from rebel territory using a system supplied by Russia.
Ukrainian officials say their forces in recent days have recaptured at least 10 towns, shrinking the amount of territory under rebel control in the embattled regions of Luhansk and Donetsk and gaining substantial advantage, including over some of the main highways in the region.
Officials have said they believe that they could defeat the rebels within three weeks if there is no further intervention by Russia, either in the form of new vehicles and weapons or — less likely — a direct invasion by Russian forces.
While Ukraine has long complained about the flow of weapons, tanks, and volunteer fighters across the Russian border, the allegations of direct Russian intervention have grown far more serious in the last two weeks.
On July 14, a Ukrainian military transport plane was shot down by what Ukrainian officials said was a powerful missile fired from the Russian side of the border.
The next day, officials said, a Russian plane bombed an apartment building in the town of Snizhne, killing 11 people, and the day after that, a Russian MiG-29 crossed into Ukrainian airspace and engaged two Ukrainian Su-25 fighter jets in a dogfight before shooting one down. The Malaysia Airlines jetliner, with 298 people aboard, was shot down the next afternoon.
A senior US official said last week that Russia had about 13,000 troops and more than half a dozen combined arms battalions near the border.
On Thursday, another US official said that more units were headed to the border area, and that it remained to be seen whether they were being sent to reinforce the Russian forces or replace some of the troops there.