The EU and US have urged Russia to stop arming Ukraine rebels after Ukraine said Russian tanks and artillery crossed the border on Thursday (12 June).

European Commission President Jose Manuel Barroso said Ukraine’s new leader, Petro Poroshenko, told him about the tanks in a phonecall.

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“It is very important that Russia ceases any kind of military actions in this area,” Barroso noted in a statement.

Jen Psaki, the US State Department spokeswoman, told press in Washington: “If these latest reports [on tanks] are true, which I don’t have confirmation of, this incursion marks a serious and disturbing escalation.”

The US ambassador to Ukraine, Geoffrey Pyatt, on Friday morning gave credence to Poroshenko’s report.

He tweeted a link to video footage showing tanks rolling down a street in rebel-held east Ukraine, adding: “Kremlin! Tanks in Donetsk – this is your 'humanitarian assistance' to Ukraine?”.

The escalation comes as the EU and Ukraine prepare to sign a free trade treaty on 27 June which will legally bind Ukraine to stay out of Russia’s Eurasian Union.

The EU and US say they will impose economic sanctions on Russia if it does not cease hostilities in the run-up to the date.

Russian President Vladimir Putin and Poroshenko also spoke by phone on Thursday in what Barroso called a “positive step”.

Meanwhile, Russian foreign minister Sergei Lavrov told press he has his own reports that Ukraine is using phosphorous bombs, which are banned by the UN.

He said Russia is to file a UN Security Council resolution calling on Ukraine to stop fighting the rebels.

But the US spokeswoman, Psaki, dismissed the move. She told the Washington press corps: “We find it hypocritical for the Russian leadership to call for an end to violence or call on the Ukrainians to take these steps when Russian separatists are abetting the violence and bringing weapons illegally into the country.”

Speaking in a separate interview with Germany’s Der Spiegel magazine, German defence minister Ursula Von der Leyen noted that Putin and Lavrov statements carry less weight than they used to.

“Vladimir Putin has destroyed an enormous amount of trust with his conduct ... Currently, Russia is not a partner. Partners adhere to joint agreements”, she said.

She noted that Germany is contributing to an increase in Nato's military presence in former Iron Curtain members: “President Putin knows that Nato stands solidly behind its eastern members. That's why he won't touch the sovereignty or integrity of these countries.”

The EU’s Barroso spoke on Ukraine while on a tour of three other former Soviet republics which are building closer EU ties.

On Wednesday and Thursday he visited Moldova and Georgia, which are to sign EU trade treaties along with Ukraine, and on Friday he went to Azerbaijan, which is to supply gas for an EU pipeline bypassing Russia.

He said the EU is spending €131 million on support for visa-free travel, energy reform, and rural development in Moldova this year. It has also doubled tariff-free import quotas on Moldovan fruit and vegetables.

He noted that closer EU ties can “benefit” Transniestria - a Russian-occupied breakaway region - if Transniestria puts “the right dispositions” in place.

With Russia threatening trade sanctions on Moldova if it signs the EU pact, Barroso added: “I call upon Russia to take advantage of the new opportunities and not to take punitive measures … There is no economic reason nor legal justification for such behaviour.”