KIEV, Ukraine — The Ukrainian government said Saturday that it had proof Russia had provided the surface-to-air missile system that shot down a Malaysia Airlines passenger jet over eastern Ukraine on Thursday, killing all 298 people aboard.

At a news conference in Kiev, Vitaly Nayda, the head of counterintelligence for the Ukrainian State Security Service, displayed photographs that he said showed the three Buk-M1 missile systems on the road to the Russian border.

Two of the devices, missile launchers mounted on armored vehicles, crossed the border into Russia about 2 a.m. Friday, or less than 10 hours after the jet, Flight 17, was blown apart in midair, he said. The third weapon crossed about 4 a.m.

Nayda said that the missile had been fired from the town of Snizhne, in rebel-controlled territory, echoing U.S. intelligence showing the missile coming from eastern Ukraine. Both the Ukrainians and the Americans said they believed the separatist rebels would have needed help from Russia in order to fire such a weapon.

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Tensions flared on several fronts Saturday with reports of heavy fighting between rebels and government forces in the eastern city of Luhansk, a reminder that the crash site is in an active combat zone. Meanwhile, the Kremlin announced it was imposing economic sanctions on 12 Americans in retaliation for a new round of economic sanctions announced last week against Russian companies.

The Kremlin has forcefully denied any role in the downing of the plane and has gone on the offensive, saying the Ukrainian military’s anti-aircraft weapons may have been responsible.

Ukrainian officials called for an international investigation.

“We have proof that the terrorist attack was planned and carried out with the involvement of representatives of the Russian Federation,” Nayda, the intelligence official, said. “We know that Russia is trying to hide its terrorist activity and their direct involvement.”

In Malaysia, where officials are grappling with the tragedy of losing a second major jetliner this year, the government has joined the call for an investigation but is reluctant to assign blame for the crash. Experts and officials said two concerns shaped the Malaysian government’s wariness: its bruising experience with confusion after the loss of Malaysia Airlines Flight 370 more than four months ago, and a desire not to alienate Russia and China, its main partner in East Asia.