Tensions between Russia and Ukraine have escalated sharply as Moscow sent more than 130 trucks rolling across the border in what it said was a mission to deliver humanitarian aid.
Ukraine called it a "direct invasion," and the US and Nato condemned the move as well.
The trucks, part of a convoy of 260 vehicles, entered Ukraine without government permission after being held up at the border for a week amid fears the mission was a Kremlin ploy to help the pro-Russian separatists in eastern Ukraine.
By late afternoon, trucks had reached the city of Luhansk, whose war-reduced population of a quarter-million people has suffered under intense fighting over the past several weeks between Ukrainian forces and the separatists.
Russia said the white-tarped vehicles were carrying food, water, generators and sleeping bags.
The arrival of the trucks instantly raised the stakes in the crisis. An attack on the convoy could give Russia a pretext to intervene more deeply in the fighting.
And the convoy's mere presence could block further battlefield advances by Ukrainian forces, which have reported substantial inroads against the rebels over the past week.
In sending in the convoy, Russia said it had lost patience with Ukraine's stalling tactics and claimed that soon "there will no longer be anyone left to help" in Luhansk, where weeks of heavy shelling have cut off power, water and phone services and made food scarce.
Ukraine has long accused Russia of supporting and arming the rebels, a charge Russia denies. Yet Nato said that, since mid-August, it has seen multiple reports of direct involvement of Russian forces in Ukraine.
It also said Russian artillery is being used against Ukraine's forces, both from across the border and from inside Ukraine. In addition, Nato said it has seen "transfers of large quantities of advanced weapons, including tanks, armoured personnel carriers and artillery, to separatists".
At the United Nations in New York, Russian ambassador Vitaly Churkin hotly denied any Russian troops were inside Ukraine.
Moscow's decision to move unilaterally, without Red Cross involvement, raised questions about its intentions.
Suspicions were running high that the humanitarian operation may instead be aimed at halting Kiev's momentum on the battlefield.
Ukrainian PM Arseniy Yatsenyuk declared that the trucks were half-empty and were not going to deliver aid but would instead be used to create a provocation. He said Russia would somehow attack the convoy itself, creating an international incident.
Ukrainian security services chief Valentyn Nalyvaichenko called the convoy a "direct invasion".
Nato's secretary-general condemned Russia for sending in a "so-called humanitarian convoy" Anders Fogh Rasmussen said Russia committed "a blatant breach" of its international commitments and "a further violation of Ukraine's sovereignty".
AP journalists following the convoy across rough country roads heard the trucks' contents rattling and sliding around, confirming that many vehicles were only partially loaded.
Mr Nalyvaichenko said the men operating the trucks were Russian military personnel trained to drive combat vehicles, tanks and artillery.
He insisted, however, that Ukraine would not shell the convoy.
The Red Cross, which had planned to escort the convoy to assuage fears that it was a cover for a Russian invasion, said it had not received enough security guarantees to do so, as shelling had continued overnight. Four troops were killed and 23 wounded in a 24-hour period in eastern Ukraine, the government reported.
The government said it had authorised the entry of only 35 trucks. But the number of Russian vehicles seen passing through was clearly way beyond that.
International monitors said that as of midday, 134 trucks, 12 support vehicles and one ambulance had crossed into Ukraine.
In announcing its decision to act, the Russian Foreign Ministry said: "There is increasingly a sense that the Ukrainian leaders are deliberately dragging out the delivery of the humanitarian load until there is a situation in which there will no longer be anyone left to help."
It added: "We are warning against any attempts to thwart this purely humanitarian mission."
Rebel forces took advantage of Ukraine's promise not to shell the convoy to drive on the same country road as the trucks. Some 20 green military supply vehicles - flatbed trucks and fuel tankers - were seen travelling in the opposite direction, along with smaller rebel vehicles.
The fighting in eastern Ukraine began in mid-April, a month after Russia annexed Ukraine's Crimean peninsula. It has killed over 2,000 people and forced 340,000 to flee, according to the United Nations.
Lithuanian foreign minister Linas Linkevicius said the country's honorary consul in Luhansk had been abducted and killed by "terrorists".