Izvaryne: The West rebuked Russia for sending scores of trucks from a controversial aid convoy to east Ukraine's rebel-held Lugansk in a move Kiev decried as an "invasion."
The European Union and the United States demanded that Russia immediately withdraw the convoy, amid fears the cargo could shore up pro-Moscow rebels fighting Kiev's forces.
Washington warned Russia it could face further sanctions, while the UN Security Council expressed concern the move could lead to an escalation in the four-month conflict.
Some 280 trucks from the Russian convoy had been waiting at the border with Ukraine for a week as Moscow pressed for the aid to be delivered urgently to civilians in areas that have come under Ukrainian shelling.
By evening, the trucks had all reached Lugansk and were unloading their cargo, Russian state television reported.
Russian President Vladimir Putin said further delay would have been "unacceptable" as he justified the decision in a phone conversation with German Chancellor Angela Merkel, who travels to Kiev Saturday for talks with Ukraine's leadership.
Merkel and US President Barack Obama warned that Moscow's military presence and shelling were triggering a "dangerous escalation" of the conflict, and they demanded it remove the convoy.
In a telephone call, the pair "agreed that Russia sending a convoy into Ukraine without Ukraine's approval is a further provocation and a violation of Ukraine's sovereignty and territorial integrity," the White House said.
A first group of trucks reached Lugansk, which has been without water and power for weeks, after making its way along a perilous route from the border, a local official said.
Ukraine's security service head Valentyn Nalyvaychenko condemned the entry as "a direct invasion" but said Ukraine will not order air strikes on the trucks.
Ukraine and Russia each said the other side was responsible for the convoy's security between the border and rebel bastion Lugansk, 63 kilometres (40 miles) away, and Russia's foreign ministry warned "against any attempts to disrupt a totally humanitarian mission."
The UN Security Council met for urgent consultations, at Lithuania's request, but Russia insisted the aid was desperately needed and accused Ukraine of using stalling tactics.
- Baby food? -
Russian Ambassador to the UN Vitaly Churkin provided a list of the aid, including electric generators, sugar, tea and baby food, and scoffed at suggestions that these could have military purposes.
Analysts said Moscow is under pressure from the Russian public to show support for the Russian-speaking separatist regions, but that its unannounced convoy gambit was a big risk.
"Now the chances of direct military confrontation between Russian and Ukrainian soldiers is substantially higher," said independent Moscow-based analyst Maria Lipman.
Moscow said it was ready to have Red Cross officials accompany the convoy, but the organisation backed out of the operation because of fierce fighting raging in the area where the trucks are heading.
NATO chief Anders Fogh Rasmussen called Moscow's decision an escalation of the Ukraine crisis that "can only lead to Russia's further isolation."
The United States urged Moscow to "immediately" withdraw the convoy. "Failure to do so will result in additional costs and isolation," Pentagon spokesman Rear Admiral John Kirby told reporters.
- Border guards blocked -
Ukraine's border service said its officials were "blocked" at the Russian checkpoint as the convoy started rolling across the border and had not checked many of the trucks.
UN officials told the Security Council in New York that only 34 of the 280 trucks had been inspected, leaving a question mark over the overwhelming bulk of the cargo.
The first vehicles to cross were met by an escort of rebels driving in minivans.
Lugansk has been under heavy shelling with civilian casualties.
Among the latest casualties was Lithuania's honorary consul, who was "kidnapped and brutally killed" by the rebels who control the city, Foreign Minister Linas Linkevicius said.
An "appalled and saddened" European Union said the incident added urgency for "Russia to immediately cease its actions destabilising Ukraine and to join our efforts to restore peace and security in Ukraine."
After four months of fighting that has cost some 2,200 lives, Ukrainian forces have been steadily gaining ground with the separatists now surrounded in strongholds and street battles erupting in populated areas.
Both Kiev and Moscow appear to be trying to strengthen their positions ahead of a meeting between Ukrainian President Petro Poroshenko and Putin on Tuesday in Minsk alongside top EU officials.
On Thursday, Poroshenko said his delegation was going to Minsk to "talk peace" but that he would stick to demands pro-Russian fighters withdraw from east Ukraine.
Kiev has accused Moscow of fuelling the insurgency that erupted after Russia annexed Crimea in March, setting off the worst crisis between Russia and the West since the Cold War.
Kiev's security forces said the military has made further gains, with the press service for the operation in the east reporting "considerable enemy losses."
But Kiev also said that pro-Russian rebels shot down a Ukrainian air force helicopter on Wednesday near Lugansk, killing the two-member crew.