Ukraine says Russian invasion has begun

He noted that he will convene a special meeting of Ukraine's security council, the NSDC.

He also called for snap talks by the UN Security Council, noting: “The world must provide an assessment of the sharp aggravation of the situation”.

Ukraine’s prime minister, Aresniy Yatsenyuk, in a separate TV statement, said: “It’s quite difficult for us to fight with Russia and its army. We urge the EU and the US to freeze all Russian assets until Russia pulls back its forces”.

Ukraine’s ambassador to the EU, Konstantin Yeliseyev, noted in a communique that EU leaders at their summit on Saturday in Brussels should devote a special session to Ukraine, adding: “Solidarity … should materialise in further resolute significant sanctions [against Russia] and large-scale military and technical assistance to Ukraine”.

Russian forces already invaded Ukraine when Russia seized Crimea in March.

The EU and the US imposed economic sanctions on Russia over Crimea and over its supply of weapons and irregular fighters to pro-Russia rebels in east Ukraine over the past five months.

But Kiev, Washington, and some EU capitals said on Thursday that Russia crossed a new line when its soldiers overtly took part in attacks against government forces in south-east Ukraine.

“We consider this as a real invasion,” a Ukrainian diplomat told EUobserver.

A diplomat from one eastern European EU country said: “Now there is a direct Russian military invasion, so the EU should react accordingly. The possibility of further sanctions will be discussed”.

Edgars Rinkevics, the foreign minister of Latvia, tweeted: “UN must react accordingly, this is war”.

For its part, the EU foreign service has said foreign ministers will hold talks on Ukraine at an informal meeting in Milan this weekend.

An EU diplomat noted that Poroshenko is due in Brussels on Saturday morning to meet with EU Council chief Herman Van Rompuy and European Commission head Jose Manuel Barroso in the margins of an EU summit devoted to filling the bloc’s top posts.

He added that EU leaders will discuss Ukraine on Saturday evening and that Poroshenko may be invited to join them depending how events unfold.

Meanwhile, Van Rompuy in his invitation letter to EU leaders said the second half of the summit will be in the presence of foreign policy chief Catherine Ashton to "discuss a number of pressing international issues."

In a separate move, the commission also on Thursday said it will pay EU producers of butter, skimmed milk powder, and selected cheeses to put excess stock into cold storage over the next three to seven months.

The decision comes after Russia imposed a ban on EU food imports earlier this month in retaliation at EU measures against its banks and energy firms.

The commission also said member states are due in October to file reports on how they would cope if Russia cuts off supplies of gas to EU customers via Ukraine in the coming winter.