Russian President Vladimir Putin visited Crimea as skepticism rises over a Russian convoy headed towards eastern Ukraine allegedly carrying humanitarian aid. The final destination of the convoy is unknown. (Reuters)

A Russian aid convoy struck a new course toward rebel-held territory in eastern Ukraine on Thursday, apparently flouting Ukrainian demands that all humanitarian shipments be subject to government inspections and prompting Kiev to threaten a direct confrontation if it tries to pass through.

“Any attempt to introduce any convoy without the agreement of Ukraine will be considered an open aggression,” Ukrainian military spokesman Andriy Lysenko told reporters in Kiev. “The entire world will consider them direct aggressors.”

It is the second time in less than a week that tensions between Ukraine and Russia have flared over the convoy, which Ukrainian authorities refused to allow through a government-controlled border crossing in the Kharkiv region earlier this week.

Early Thursday, the vehicles — which had been idling at a military depot — set off on their journey south through Russian territory along a highway leading from Moscow through areas abutting Ukraine’s eastern regions. By afternoon, the convoy had taken a sharp turn westward and was headed directly for the Izvarino border crossing in the Luhansk region, which is controlled by pro-Russian separatists.

Russian officials have maintained that the convoy of more than 250 trucks is a humanitarian mission, intended to provide the beleaguered civilians of war-torn Luhansk with food, blankets and other emergency supplies. But Ukrainian officials fear that Russia might use the mission as a Trojan Horse to launch a full-scale invasion of eastern Ukraine.

Late Monday, Ukrainian President Petro Poroshenko agreed to let Russian and European aid be sent to Luhansk under the auspices of the International Committee of the Red Cross. Russia dispatched the convoy hours later, saying it was cooperating with the ICRC and complying with the agreement. But Ukrainian and ICRC authorities said they had never certified the shipment.

Journalists following the convoy were allowed to inspect the vehicles Thursday on the road to Luhansk and saw large amounts of food supplies. But a reporter for the Financial Times also noted the presence of two heavy military vehicles on flatbed trucks bringing up the rear of the convoy.

The prospect of a direct confrontation between the Russian convoy and the Ukrainian military appeared to grow Thursday as Kiev announced that troops had seized the town of Novosvitlivka, located along the main highway between the city of Luhansk and the rebel-held checkpoint closest to the location of the convoy. The Washington Post could not independently verify the government’s account.

But should the convoy pass the Ukrainian military without having been checked by border guards, Kiev will have to make the difficult decision of whether to fire on the trucks, risking a Russian invasion, or acquiesce to what would amount to a major violation of Ukraine’s sovereignty.

As Ukraine and Russia continued to negotiate with the ICRC over the Russian convoy, Ukraine’s government on Thursday started sending its own aid shipments to the east, dispatching 75 trucks loaded with supplies. The aid was sent even as Kiev significantly escalated the bombardment of the rebel stronghold Donetsk, where city officials said two civilians were killed after a central boulevard was shelled.

Rinat Akhmetov, a steel magnate and one of eastern Ukraine’s most powerful oligarchs, also announced that he would begin sending humanitarian aid to the Donetsk and Luhansk regions next week.

Although no white flags are yet waving in those regions, leading rebel figures in Donetsk and Luhansk stepped down from their positions Thursday. Valery Bolotov cited injuries as his reason for passing the leadership of the Luhansk rebels to their self-proclaimed “defense minister,” Igor Plotnitskiy. In Donetsk, chief defense leader and Russian citizen Igor Girkin — better known by his nom du guerre Igor Strelkov — also stepped down, according to a report from the Russian news service Interfax. A Donetsk rebel spokesman quoted in the report dismissed rumors that Girkin had recently been injured as “a hoax.”

Michael Birnbaum in Kiev contributed to this report.