The UN Security Council met for emergency consultations after Russia sent an aid convoy rolling into eastern Ukraine, where pro-Moscow rebels are fighting Kiev's forces.
Lithuanian Ambassador Raimonda Murmokaite, whose country requested the urgent meeting, said the incursion of scores of Russian trucks into Ukraine without Kiev's consent was a "violation of Ukraine's sovereignty."
Ukraine described the convoy as an "invasion" while UN Secretary General Ban Ki-moon voiced his deep concern and called for restraint.
Council members began the talks behind closed doors on Friday (local time) and were to hear a briefing from senior UN official Oscar Fernandez-Taranco on the convoy that the West fears could be used to help separatist forces.
The United States and European governments had urged Moscow to work with Ukrainian authorities and the International Red Cross to verify the content of the cargo and ensure it arrives safely to civilians.
In Washington, the Pentagon demanded that Russia immediately pull out its trucks and warned of additional sanctions.
At a news conference, Ukrainian Deputy Ambassador Oleksandr Pavlichenko called the Russian convoy a "blatant violation of our sovereignty" and said Kiev was not aware of the content of the cargo.
Ukrainian officials were blocked Friday from inspecting the content, according to the Ukrainian foreign ministry, and the Red Cross decided against accompanying the convoy due to security concerns.
Russian Ambassador Vitaly Churkin however provided a list of the aid including electric generators, sugar, tea and baby food, and scoffed at suggestions that these could have military purposes.
Asked about the aid helping the rebels, he responded: "With baby food?"
"The US do not have the monopoly on humanism," Churkin said. "If you are trying to question our humanism, I would resent that."
He insisted that Russia ordered the convoy to move after multiple attempts at reaching agreement with Ukraine for the deliveries failed.
"It was time to move and so we did," said the ambassador.
Churkin said he hoped that the Red Cross would help with the distribution of the aid to besieged civilians who have been without water and electricity for weeks amid an escalation of fighting.
Around 20 trucks entered the rebel bastion of Lugansk on Friday (local time), part of the nearly 300 lorries that have been waiting at the Russian border for a week.
Ban said that while there was a need for humanitarian aid, "any unilateral action has the potential of exacerbating an already dangerous situation in eastern Ukraine."
Kiev has accused Moscow of fuelling the insurgency, which erupted after Russia annexed Crimea in March, setting off the worst crisis between Russia and the West since the Cold War.