U.S. President Barack Obama and German Chancellor Angela Merkel have expressed alarm at Russian moves inside Ukraine.
According to the White House, Obama and Merkel said the presence of Russian soldiers in Ukraine, the buildup of Russian troops along the Ukrainian border, and Russian shelling into Ukraine represent dangerous escalations of tensions by Moscow.
The White House says the two leaders agree that a Russian convoy that entered Ukraine without approval is yet another provocation by Moscow that violates Ukraine's sovereignty.
The two leaders spoke by telephone ahead of a scheduled visit by Merkel to Kyiv later on August 23.
Russia says the convoy that entered Ukraine on August 22 is for humanitarian purposes, but Ukraine and the United States say the Russians failed to abide by conditions set by Ukraine and the International Red Cross.
Obama and Merkel agree that Russia must remove the convoy and withdraw from Ukrainian territory.
Earlier, Ben Rhodes, Obama's deputy national security adviser, said Russia would face additional costs if the convoy is not removed. He said the United States would discuss the matter on August 22 with its partners on the UN Security Council.
In that closed-door emergency meeting at the UN, several countries rebuked Russia for "what many called an illegal and unilateral action by the Russian federation," British Ambassador Mark Lyall Grant, the council president, told reporters.
Russian Ambassador Vitaly Churkin said he defended his country during the discussions, reminding his colleagues that Moscow had received an "official note" from Kyiv welcoming the idea of the Russian convoy.
He reiterated Russia's stance that Ukrainian border guards were deliberately stalling the convoy.
UN Secretary-General Ban Ki-moon warned that "any unilateral action has the potential of exacerbating an already dangerous situation in eastern Ukraine."
'Trucks Half Empty'
In Brussels, NATO Secretary-General Anders Fogh Rasmussen called Russia's unilateral decision to send more than 130 trucks filled with what it called humanitarian aid into rebel-held areas "a blatant breach of Russia's international commitments."
NATO also said Russian artillery is being used against Ukrainian forces, both from across the border and from inside Ukraine.
NATO also said it has seen "transfers of large quantities of advanced weapons, including tanks, armored personnel carriers and artillery, to separatists."
Ukrainian Prime Minister Arseniy Yatsenyuk said the trucks were half empty and were not aiming to deliver aid but to create a provocation.
AP reporters following the convoy said they heard the trucks' contents rattling and sliding around, suggesting that many of the vehicles were only partially filled.
AP also reported rebel forces took advantage of Kyiv's promise not to shell the convoy to drive on the same country roads as the trucks.
The agency said some 20 green military supply vehicles were seen traveling in the opposite direction.
In other news, Lithuanian Foreign Minister Linas Linkevicius said the country's honorary consul in Luhansk had been abducted and killed by "terrorists."
There were no further details.
Based on reporting by Reuters and AP