The US has threatened Russia with more sanctions after accusing Moscow of violating Ukraine’s sovereignty by sending an aid convoy into the east of the country.
Russia, which has thousands of troops close to its side of the border, warned against any attempt to “disrupt” the convoy, which it said was purely providing humanitarian aid to rebel-held areas.
The vehicles are carrying water, generators and sleeping bags reportedly intended to help civilians in Luhansk, where pro-Russian separatists are besieged by Ukrainian government forces.
The trucks, part of 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.
NATO Secretary General Anders Fogh Rasmussen said the convoy looked like a disguised attempt to reinforce the pro-Moscow forces.
The alliance also said Russian troops have been firing artillery across the border and within Ukraine since the middle of August, a de facto charge that Moscow is already waging war.
White House deputy national security adviser Ben Rhodes warned of “additional costs” if the convoy was not withdrawn.
When asked about the NATO claim, he said: “We have seen the use of Russian artillery in Ukraine in the past days.”
Russia denies backing the rebels militarily.
Ukrainian President Petro Poroshenko described the development as a “flagrant violation of international law”.
However, a senior security chief said Ukrainian forces would let them pass to avoid “provocations”.
“We consider this a direct invasion by Russia of Ukraine,” Ukrainian state security chief Valentyn Nalivaychenko said.
Asked whether Ukraine would use air strikes against the convoy, he said: “Against them, no.”
In a telephone call with German Chancellor Angela Merkel, President Vladimir Putin said Russia had been unable to wait any longer for the green light from Ukraine to help people in distress.
US President Barack Obama spoke with Ms Merkel, and the pair agreed sending the trucks across the border was a “provocation” and called on Moscow to remove the convoy.
They said Russia was triggering a “dangerous escalation” of the conflict due to its military presence and shelling.
Mr Poroshenko said more than 100 vehicles had crossed into Ukraine, and only some had been checked by Ukrainian officials inside Russia.
Other Ukrainian officials said only 34 or 35 of the trucks had been properly checked.
The International Committee of the Red Cross, which both Russia and Ukraine had agreed should supervise the convoy, said it was not escorting it “due to the volatile security situation”.
A separatist rebellion broke out in the east of Ukraine soon after Russia annexed the Crimean peninsula in March.
The last few weeks have seen a string of rebel defeats in a conflict that has killed more than 2,000 people.