The United Nations Security Council is holding emergency consultations after Russia sent an aid convoy rolling into eastern Ukraine.
Lithuania requested the meeting to discuss the entry of scores of trucks that Ukraine has described as an "invasion”.
The Pentagon has said the US is very concerned about the movement of the convoy into the Ukraine and has urged Moscow to withdraw it immediately.
"Russia must remove its vehicles and its personnel from the territory of Ukraine immediately. Failure to do so will result in additional costs and isolation," Pentagon spokesman Rear Admiral John Kirby told reporters.
"Russia should not send vehicles, persons, or cargo of any kind into Ukraine, whether under the guise of humanitarian convoys or any other pretext, without Kiev's express permission," he said.
NATO chief Anders Fogh Rasmussen sharply criticised the Russian decision to advance a "so-called" aid convoy into Ukraine on Friday as a "blatant breach" of international commitments that "only deepens the crisis".
"This is a blatant breach of Russia's international commitments... and a further violation of Ukraine's sovereignty by Russia," Mr Rasmussen said in a statement.
He poured doubt on Moscow's claim that the convoy, which had been waiting at the border for a week, was actually meant for humanitarian purposes.
Ever since Russia annexed Crimea in March, NATO has taken a leading role in Western accusations that Moscow was actively involved in destabilising eastern Ukraine.
The transatlantic military alliance accuses Moscow of actively providing weapons and fighters to pro-Kremlin separatists as well as massing its own troops near the border.
Putin tells Merkel further aid delays to Ukraine 'unacceptable'
Russian President Vladimir Putin has told German Chancellor Angela Merkel that further delays in delivering humanitarian aid into eastern Ukraine would have been unacceptable, the Kremlin said in a statement.
In a phone call initiated by the German leader, Mr Putin said that following "clear procrastination" by Kiev, Russia decided to begin the controversial mission because "further delays would have been unacceptable" given the humanitarian catastrophe in the conflict zone, the Kremlin said.
After a week-long delay at the border, a Russian convoy of 280 trucks carrying 1,800 tonnes of humanitarian aid according to Moscow rolled into Ukraine without being inspected by Ukrainian authorities or accompanied by the Red Cross.
Kiev called the action an invasion and the EU a "clear violation" of Ukraine's border.
The Russian foreign ministry rebuffed accusations that Moscow had violated international law.
"We are acting in full accordance with the norms of international humanitarian law," Deputy Foreign Minister Sergei Ryabkov said in a statement.
He alleged that Kiev had wanted to block the delivery of the aid while it tried to achieved a military victory.
"It was Kiev, clearly backed by its Western supporters, who did everything to derail this important humanitarian action," said Ryabkov.
"The Ukrainian authorities needed time to try to complete the military operation to suppress the protest of its own people, spilling blood and tears where Russian humanitarian aid is now being distribution," he said. "They didn't succeed."
Local officials confirmed the convoy had arrived in Lugansk today.
Fighting between Ukrainian government forces and pro-Russian rebels has recently grown more deadly around Lugansk, the decimated former home to nearly 420,000 mostly Russian-speakers.
Many have fled due to a dire lack of food, water and medicine.
Lithuania's honorary consul in eastern Ukraine killed
Lithuania's honorary consul in eastern Ukraine's Lugansk has been kidnapped and killed by the pro-Russian rebels there, according to Lithuania's Foreign Minister Linas Linkevicius.
"With deep sorrow just learned about Lithuania's honorary consul in Lugansk Mr Mykola Zelenec kidnapped and brutally killed by terrorists there," Mr Linkevicius, who is on a visit to Kiev, wrote on his Twitter blog.
Earlier, the Ukrainian government said that pro-Russian rebels shot down a Ukrainian air force helicopter near Luhansk this week, killing the two-member crew.
The Mi-24 helicopter was shot down on Wednesday said security spokesman Andriy Lysenko, the same day a Ukrainian war plane was downed near the city.