Ukraine crisis: Russian army vehicles seen near border
Around a dozen Russian light tanks have been seen heading for the Ukrainian border, as a Russian aid convoy remains parked near the frontier.
The BBC saw the tanks early on Friday morning, but there was no confirmation that they were going to Ukraine.
Two other reporters say they have seen Russian army vehicles entering Ukraine.
The convoy is still awaiting permission to take its cargo to cities held by pro-Russian rebels in eastern Ukraine which have been wracked by violence.
Red Cross officials are in the Ukrainian capital Kiev for talks on the convoy.
Ukraine, which fears the convoy may carry military supplies for the rebels, has threatened to block it unless it is independently checked.
Russia's government has consistently denied directly arming or training pro-Russian rebels, who sparked the conflict in April when they took control of several cities in eastern Ukraine.
The conflict, which has claimed more than 2,000 lives, has intensified in recent weeks.
On Thursday, there was heavy shelling in the rebel stronghold cities of Donetsk and Luhansk.
And the rebels announced that their main military commander Igor Girkin - known as Strelkov - had resigned.
The humanitarian convoy of at least 260 lorries drove for nine hours on Thursday before parking in a field near the border.
Russia dismissed as absurd claims that its convoy was a pretext to send military supplies to the rebels.
Red Cross official Laurent Corbaz, in Kiev to discuss the convoy, said the Red Cross had a "strictly humanitarian role" and that "the delivery of aid should not be politicised".
Red Cross spokeswoman Anastasia Isyuk said the convoy was "south of the city of Kamensk-Shakhtinski" and that the Red Cross had been in contact with the Russian representatives.
She said there was still no agreement on the issues of border crossing procedures and customs clearance.
The BBC's Steve Rosenberg, who has been following the convoy, says the key question now is what Russia will do next - if it takes the convoy across the border, it will be seen by the Ukrainian authorities as a major provocation.
Ukraine is sending its own 75-lorry aid convoy to the east and Mr Corbaz said that too was being discussed in Kiev.
The US has issued another warning to Russia.
State department spokeswoman Marie Harf said: "We've made that very clear to the Russians that they should not move these trucks in, without taking all of the steps the Ukrainian government has outlined."
Heavy fighting continued on Thursday, with intense artillery shelling in both Luhansk and Donetsk.
Ukrainian authorities said they had cut off Luhansk from other rebel-held areas after capturing the town of Novosvitlivka.
Ms Harf said the US had "stressed the importance of showing restraint to minimise casualties among the civilian population".
The Russian foreign ministry on Thursday called for an "urgent" ceasefire.
The loss of Strelkov, meanwhile, represents the third high-profile resignation of rebel leaders in the past week.
Some 2,086 people have been killed since the conflict in the east began in mid-April, more than half of them in the past two weeks, the UN says.
The violence began when pro-Russian rebels seized government buildings and tried to declare independence.
The military launched an operation to retake the east, and stepped up its activities in June.
Russian President Vladimir Putin on Thursday visited Crimea, which Russia annexed from Ukraine in March in a move that drew international condemnation.
Mr Putin said Russia's goal was "to stop bloodshed in Ukraine as soon as possible".
He said Russia should not "fence itself off from the outside world" although he said Russia would "not allow anyone to treat us with arrogance".
On Thursday it emerged that Russia's largest oil company, Rosneft, had asked the Russian government for more than $40bn (£24bn) in order to help it weather the impact of Western sanctions.