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The White House joined Kiev tonight in accusing Russia of a "flagrant violation of international law" after a convoy of lorries carrying humanitarian aid crossed the border with Ukraine without permission.
It said Russia should remove them immediately, or face further economic sanctions.
ITV News' Europe Editor James Mates reports from Washington:
UN Secretary-General Ban Ki-moon has said he is deeply concerned at reports of a Russian aid convoy entering Ukraine without permission from the nation's government.
Ban's spokesman Stephane Dujarric said in a statement: "While recognizing the deteriorating humanitarian situation, any unilateral action has the potential of exacerbating an already dangerous situation in eastern Ukraine."
The White House has called a Russian convoy's move into Ukraine a "flagrant violation" of the country's sovereignty and urged Russia to remove it or face more economic sanctions.
White House deputy national security Ben Rhodes told reporters that sending the convoy into Ukraine adds to the escalation of the conflict "in a dangerous way."
The United States plans to discuss the situation with the UN Security Council later today, he added.
Europe minister David Lidington has warned Russia that it risks "further damaging consequences" if it continues to escalate tensions in Ukraine.
Mr Lidington said that the entry of a Russian convoy into Ukraine is a "provocative act that violates previous understandings".
The Pentagon has accused Russia of violating Ukraine's sovereignty by sending an aid convoy into the country, and warned that it must withdraw all vehicles and personnel "immediately".
Rear Admiral John Kirby, the Pentagon's press secretary, said in a press conference:
– Rear Admiral John Kirby, pentagon
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.
This is a violation of Ukraine's sovereignty and territorial integrity by Russia.
Russia must remove its vehicles and its personnel from the territory of Ukraine immediately. Failure to do so will result in additional cost and isolation.
NATO Secretary General Anders Fogh Rasmussen has spoken of an alarming build-up of Russian ground and air forces close to the border with Ukraine.
He condemned Russia for sending a "so-called humanitarian convoy" into Ukraine without the consent of the Ukrainian authorities.
Russian President Vladimir Putin has reportedly told German Chancellor Angela Merkel that he decided to send a humanitarian aid convoy into Ukraine because he could no longer wait.
According to the Kremlin account of the phone call, President Putin expressed "serious concern" about the military escalation in eastern Ukraine.
Ukrainian President Petro Poroshenko has said the entry of more than 100 Russian aid trucks into the country is a "flagrant violation of international law."
In a statement on his website, he added none of the trucks' cargo had been checked before being driven in.
Ukraine will not use any force against the Russian aid convoy which has now crossed into the country despite claiming they were treating it as a "direct invasion".
Ukrainian security chief Valentyn Nalivaychenko said they wished to "avoid provocations".
The movement of aid trucks into Ukraine represents a "direct invasion," Ukrainian state security chief Valentyn Nalivaychenko said.