A column of Russian armoured personnel carriers has been seen crossing into eastern Ukraine despite official denials of a military presence.
A Guardian journalist photographed one of the 20 vehicles, which carried official Russian military plates, travelling towards Ukraine from the Russian town of Donetsk, 130 miles from the rebel-held Ukrainian city with the same name.
The convoy reportedly waited for nightfall before entering Ukraine along a dirt track through a gap in the border’s barbed wire fence.
Andriy Lysenko, a spokesman for Ukraine's security council, also claimed the Russian military vehicles crossed into the country- a charge Russia denied.
Philip Hammond, the Foreign Secretary, said he was "very alarmed" by the reports.
"If there are any Russian military personnel or vehicles in eastern Ukraine they need to be withdrawn immediately or the consequences could be very serious," he added.
Erm ok so this isn't humanitarian aid. Column of over 20 APCs, 10km from the Ukraine border, and heading closer pic.twitter.com/OMvJmHzsx1— Shaun Walker (@shaunwalker7) August 14, 2014
Nato's Secretary General, Anders Fogh Rasmussen, later said there had been a "Russian incursion" into Ukraine but stopped short of characterising it as an invasion.
Russia's Federal Security Service said Russian forces were patrolling the border area but denied that military vehicles had moved in.
The Ukrainian administration is likely to view the personnel carrier sightings as confirmation of their long-held conviction about a Russian military incursion and the country's support for militants.
Government forces have not controlled the area for months, losing it to the self-proclaimed Donetsk People’s Republic and other pro-Russian paramilitary groups.
In April, Crimea was annexed with ease by the Kremlin, after a disputed referendum and the occupation of the peninsula by pro-Russian forces.
A German government spokesman, Steffen Seibert, said the accusation of Russian military vehicles crossing the border was very serious.
"It is very clear that if this report turns out to be true, Russia would be urgently called upon to withdraw these vehicles across its border at once," he said.
Ukraine crisis: Russian 'aid' convoy
Ukraine crisis: Russian 'aid' convoy
1/11 Ukraine crisis
Drivers of the first trucks of the Russian aid convoy parked in the city of Luhansk on 22 August
2/11 Ukraine crisis
An employee inspects the contents of a truck with Russian humanitarian aid in Mariupol, Ukraine on 22 August 2014
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The convoy nearing the border before it parked at a camp in Russia
4/11 Ukraine crisis
Russian convoy carrying humanitarian aid for residents in rebel eastern Ukrainian regions moving along a road in the city of Voronezh, about 530 km from Moscow, Russia
5/11 Ukraine crisis
An Ukrainian border guard checks passing cars at a checkpoint of Pletnyovka, Kharkiv region on Ukraine-Russia border, where Russian humanitarian convoy is to cross the border
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Ukrainian border guards stand at the Ukrainian-Russian border crossing
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Trucks of a Russian convoy carrying humanitarian aid for Ukraine are parked at the military air base outside Voronezh
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Drivers of a Russian convoy of trucks carrying humanitarian aid for Ukraine take a rest on a side of a road near the city of Yelets
9/11 Ukraine crisis
An Ukrainian soldier stands guard at a checkpoint of Pletnyovka, Kharkiv region on Ukraine-Russia border, where Russian humanitarian convoy is to cross the border
10/11 Ukraine crisis
A Russian convoy carrying humanitarian aid for residents in rebel eastern Ukrainian regions moves along a road about 50 km from Voronezh, Russia, 14 August 2014. The convoy continues to advance through Russian territory after a one-day stop in Voronezh in full coordination with and under the aegis of the Red Cross, according to Russian authorities
11/11 Ukraine crisis
The Russian convoy carrying humanitarian aid in the Voronezh region of Russia en route to Ukraine
Ukrainian politicians had already raised concerns about an aid convoy of more than 200 lorries due to arrive on Friday.
A tentative agreement has seen the shipment enter through a government-controlled checkpoint near Donetsk and inspected by the international Red Cross.
Ukraine said its 41 border service officials and 18 customs workers began the inspection on Friday in the presence of aid agency representatives.
Video: Fighting enters Donetsk
Russian news agencies said Russia was prepared to present all necessary documentation and to hand over the cargo to the Red Cross.
Such an inspection would ease concerns that Russia could use the aid shipment as cover for a military incursion in support of the separatists.
Russian officials said it was full of vital supplies ranging from baby food to sleeping bags for civilians facing crisis after months of fighting between government troops and militias.
Kiev has also announced its own aid shipment to the war-ravaged region of Luhansk, where almost 250,000 people have been without water or power for nearly two weeks.
The conflict has killed more than 2,080 people, according to the UN's human rights office in Geneva, and left many more desperately short of food, water and medical aid.
A spokesperson said at least 4,953 others have been wounded in the fighting since mid-April, with more than 60 people a day being killed or wounded as part of "a clear escalating trend" of violence.
Additional reporting by AP