Col. Andriy Lysenko, a spokesman for Ukraine's National Security Council, told reporters that the column of 10 tanks, two armored vehicles and two trucks crossed the border near Shcherbak and that the nearby city of Novoazovsk was shelled during the night from Russia. He said they were Russian military vehicles bearing the flags of the separatist Donetsk rebels.

In Moscow, Russian Foreign Minister Sergey Lavrov said Monday he had no information about the column.

The reported incursion and shelling could indicate an attempt to move on Mariupol, a major port on the Azov Sea, an arm of the Black Sea. Mariupol lies on the main road between Russia and Ukraine's Black Sea peninsula of Crimea, which Russia annexed in March. Capturing Mariupol could be the first step in building a slice of territory that links Russia with Crimea.

Although Mariupol is in Ukraine's separatist Donetsk region, most of the fighting between separatist rebels and Ukrainian troops has been well to the north, including around the city of Donetsk, the rebels' largest stronghold. A full offensive in the south could draw Ukrainian forces away from the fight for Donetsk.

Lysenko said Mariupol has enough defenders "to repel any attack of uninvited guests."

Ukraine and the West say that Russia is supporting and supplying the rebels and that since mid-August, Russia has fired into Ukraine from across the border and from within Ukrainian territory. Moscow denies those allegations.

Fighting continued elsewhere in the east, notably around the town of Olenivka, 25 kilometers (15 miles) south of Donetsk. Lysenko said Monday about250 separatists had been killed in that fighting, but did not specify in what time period. On Sunday, rebel leader Alexander Zakharchenko said two-thirds of Olenivka had been wrested away from Ukrainian control.

Ukrainian forces had made significant inroads against the separatists in recent weeks, but the rebels have vowed to retake lost territory.

Russia announced plans, meanwhile, to send a second aid convoy into rebel-held eastern Ukraine, where months of fighting have left many residential buildings in ruins.

Russia's unilateral dispatch of over 200 trucks into Ukraine on Friday was denounced by the Ukrainian government as an invasion and condemned by the United States, the European Union and NATO. Even though the tractor-trailers returned to Russia without incident on Saturday, the announcement of another convoy was likely to raise new suspicions.

Lavrov said Monday that Russia had notified the Ukrainian government it was preparing to send a second convoy along the same route in the coming days, but Lysenko said he had no information on that plan.

Lavrov also said the food, water and other goods delivered to the hard-hit rebel city of Luhansk was being distributed Monday and that Red Cross workers were at talks on how best to distribute it. There was no immediate confirmation on that from the Red Cross.

In sending in the first convoy, Russia said it had lost patience with what it called Ukraine's stalling tactics. It claimed that soon "there will no longer be anyone left to help" in Luhansk, where weeks of heavy shelling have cut off power, water and phone service and made food scarce.

The Ukrainian government had said the aid convoy was a ploy by Russia to get supplies to the rebels and slow down government military advances.

On Sunday, as Ukraine celebrated the anniversary of its 1991 independence from Moscow, President Peter Poroshenko announced that the government would be increasing its military spending in a bid to defeat the rebels.

In rebel-held Donetsk, captured Ukrainian soldiers were paraded Sunday through the streets, jeered by the crowd and pelted with eggs and tomatoes.