Ukraine has claimed 90 trucks from a Russian aid convoy have crossed into the country without permission in what it calls a "direct invasion".
It had been agreed the lorries would only be allowed into eastern Ukraine if they were escorted by the International Red Cross. However the charity has pulled out after not receiving enough security guarantees as fighting continues.
The vehicles are carrying water, generators and sleeping bags reportedly intended to help civilians in the city of Luhansk, where pro-Russian separatist fighters are besieged by Ukrainian government forces.
The trucks, among a 280-lorry convoy, had been held at a border crossing for a week amid suspicions by Kiev that the mission was being used as a cover for an invasion by Moscow.
But dozens of the Russian vehicles have now moved into the east of the country despite both sides in the months-long conflict ignoring pleas for a ceasefire.
"They passed into Ukraine without clearance or participation of the International Red Cross or (Ukrainian) border guards," said military spokesman Andriy Lysenko.
"We consider this a direct invasion by Russia of Ukraine," Ukrainian state security chief Valentyn Nalivaychenko said.
He added that "under the cynical guise of the Red Cross these are military vehicles."
He said Ukraine will not use any force against the convoy and wants to avoid "provocations".
Ukraine has been reluctant to let the convoy through over fears it is carrying supplies for the rebels, despite Moscow's insistence the goods were simply humanitarian aid.
'Strong army'
The transit comes as the Ukrainian president said he would tell Russia's Vladimir Putin to rein in pro-Moscow separatists, when the two leaders meet next week.
Petro Poroshenko said he would tell President Putin that Ukraine had "a strong country, a strong army" behind him.
He said "in order to have solid positions in peace negotiations, we have to be strong, to have the unity of the people, a strong country, a strong army."
"We are capable of defending our sovereignty, our independence and our territorial integrity - we are fighting for the independence of Ukraine. Together we will win for sure."
Mr Poroshenko spoke as government forces, despite taking heavy losses themselves, moved deeper into rebel-held eastern territory, putting pressure on separatists.
After a faltering start in April, government forces' success has alarmed some Western leaders who believe Mr Putin is being boxed in a corner.
German Chancellor Angela Merkel is scheduled to visit Kiev on Saturday in support for Mr Poroshenko. However diplomatic sources said she will also push Ukraine for a ceasefire to prevent a Putin backlash.
Fierce fighting has continued in the east of the country, with 16 Ukrainian special forces troops killed in fighting on Wednesday night in the town of Ilovaisk, near Donetsk.
