KIEV, Ukraine — Responding to strong advances by the Ukrainian military against separatist insurgents in the week since the downing of a passenger jet, Russia has stepped up its direct involvement in the hostilities in eastern Ukraine, Ukrainian and U.S. officials say, unleashing artillery attacks on Ukrainian positions from Russian territory and amassing heavy weapons along the border that Kiev and its allies fear could be pushed across to reinforce the rebels and prolong the fighting.
U.S. officials, citing military intelligence, including satellite images, warned that Russia appeared to be preparing to arm the rebels with more high-powered weaponry — including tanks and armored vehicles — than it had previously supplied. In Kiev, a military spokesman said 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 Friday that 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 at a briefing in Kiev, the Ukrainian capital. “We have facts on the violation of air border between Ukraine and Russia.”
A NATO military officer, who asked not to be named 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.”
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 the passenger jet, a Boeing 777 flying as 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.
In recent days, officials have said, there have been incessant attacks from Russian territory, as well as incursions into Ukrainian airspace by Russian military aircraft. Lysenko said Thursday that several border posts, along with the Luhansk airport, were shelled by Grad rockets fired from Russian territory.