KIEV, Ukraine — Rather than backing down after last week’s downing of a civilian passenger jet, Russia appears to be intervening more aggressively in the war in eastern Ukraine in what American and Ukrainian officials call a dangerous escalation that will almost certainly force more robust retaliation from the United States and Europe.

Russia has increased its direct involvement in fighting between the Ukrainian military and separatist insurgents, moving more of its own troops to the border and preparing to arm the rebels with ever more potent weapons, including high-powered Tornado rocket launchers, American and Ukrainian officials said on Friday.

The officials, citing satellite images and other military intelligence, said that Russia had positioned heavy weapons, including tanks and other combat vehicles, at several points along the border where there has been intense fighting. On Thursday, Russia unleashed artillery attacks on eastern Ukraine from Russian territory, officials in Washington and Kiev said. While Russia flatly denied accelerating its intervention on Friday, American and Ukrainian officials said Moscow appeared anxious to stem gains by government forces that have succeeded in retaking some rebel-held territory.

The reported Russian moves raised the prospect of a new and more perilous chapter opening in a conflict that has already inflamed the region and, with the destruction of Malaysia Airlines Flight 17 with 298 aboard, stunned the world. American officials blamed a Russian-provided surface-to-air missile for the explosion and hoped the shock of the episode would prompt the Kremlin to rethink its approach, but they are increasingly convinced it has not.