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KYIV, Ukraine: A convoy of Russian aid trucks that had entered Ukraine without permission returned to Russia on Saturday, but Ukrainian officials continued to express alarm, saying that Russians had loaded sophisticated military equipment onto the vehicles before they left.

Col. Andriy Lysenko, Ukraine’s military spokesman, said that trucks that had driven into the territory “under the guise of humanitarian convoys” had crossed back into Russia on Saturday morning after being packed with Ukrainian-made equipment used to produce an advanced aircraft-tracking system, as well as ammunition for small arms.

Russia’s defence sector, which has been hurt by European sanctions, has long been dependent on industries in Ukraine to provide parts for everything from planes to missiles. But the new charges added to tensions over the convoy. Earlier, Western officials had expressed suspicions that the trucks were bringing in small arms to replenish the rebels’ supplies, an allegation Russia has denied.

Russia sent the aid convoy into Ukraine without the approval of the Ukrainian government or the support of the International Committee of the Red Cross (ICRC), a move the Ukrainian authorities called a “direct invasion” and a “flagrant violation of international law.” Workers on the aid trucks, which had previously been stopped at the border for days, distributed food and other supplies Friday evening to the besieged city of Luhansk, part of a shrinking territory held by pro-Russian rebels in the face of an offensive by Ukrainian troops.

The latest incidents came as Nato said that Ukrainian troops were coming under Russian artillery fire from within this country’s borders Friday — its strongest denunciation of Russia’s role to date. Meanwhile, German Chancellor Angela Merkel arrived in Kyiv on Saturday for talks with Ukrainian President Petro Poroshenko.

Nikolai Sinitsyn, spokesman for the Russian federal border service in the Rostov region, said the disputed aid trucks had been unloaded and “already returned to Russia,” according to the Russian news service Interfax. The trucks’ speedy exit from Ukraine would be roughly in keeping with the timeline of the ICRC’s plans to have the trucks deliver emergency supplies and immediately go back to Russia.

Russia’s Foreign Ministry hinted on Saturday that the country planned to send more humanitarian aid.

“We confirm the intention to continue cooperation with the ICRC in efforts to provide humanitarian assistance to the residents of southeastern Ukraine, the need for which has by no means disappeared,” the ministry statement said. “Our assistance is still in demand.”

On Friday, the White House condemned the Russian action.

“At the same time as Russian vehicles violate Ukraine’s sovereignty, Russia maintains a sizeable military force on the Ukrainian border capable of invading Ukraine on very short notice,” National Security Council spokeswoman Caitlin Hayden said.

US Air Force Gen. Philip M. Breedlove, the supreme allied commander in Europe, condemned “Russia’s illegal incursion” into Ukraine as he expressed concern about the massing of 20,000 Russian “combat-ready troops” on the border with eastern Ukraine and the flow of Russian arms and operatives to pro-Moscow separatist forces.

The unauthorized convoy “indicates that Russia is more interested in resupplying separatists rather than supporting local populations,” Breedlove said in a statement.