A column of Russian tanks and armoured vehicles has crossed into Ukraine in the far south east, a Ukrainian official said.

Colonel Andriy Lysenko, a spokesman for Ukraine's National Security Council, said the column of 10 tanks, two armoured vehicles and two trucks crossed the border near Shcherbak.

It is away from where most of the intense fighting has been taking place but comes after the nearby city of Novoazovsk was shelled during the night from Russia.

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.

Russian foreign minister Sergey Lavrov said he has no information about the column.

It comes after Russia announced plans to send a second aid convoy to rebel-held eastern Ukraine.

It unilaterally sent about 200 tractor-trailers across the border on Friday, a move that Ukraine said was an invasion.

All of those vehicles have returned to Russia after delivering goods to the hard-hit rebel stronghold of Luhansk.

Mr Lavrov said Russia has notified the Ukrainian government that it is preparing to send a second convoy along the same route in the coming days.

Mr Lysenko said the column was made up of Russian military vehicles bearing flags of the separatist Donetsk rebels.

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

Ukraine has always said Russia is funding the rebels, a statement Russia has denied repeatedly.