REUTERS - The United States demanded Moscow remove an aid convoy it sent into rebel-held eastern Ukraine without permission on Friday, accusing Russia of a flagrant violation of the sovereignty of its former Soviet neighbour and threatening more sanctions.

Moscow, which has thousands of troops close to the Russian side of the border, warned against any attempt to "disrupt" the convoy, which it said was purely humanitarian. It did not say what action it might take if the Ukrainian military intervened.

NATO's top military commander said the movement of trucks looked like a disguised attempt to reinforce separatist forces.

The Western defence alliance said Russian troops had been firing artillery across the border and within Ukraine in a major escalation of military support for pro-Moscow rebels since mid-August, a de facto charge that Russia was already waging war.

White House deputy national security adviser Ben Rhodes said that if the convoy was not pulled out, the Russians would face "additional costs".

"We have seen the use of Russian artillery in Ukraine in the past days," he said, when asked about the NATO statement.

Moscow denies backing the rebels militarily but the United States and European Union have imposed sanctions and the Kremlin has retaliated, renewing some of the chill of the Cold War. NATO has deployed extra troops in member states bordering Russia, including former Soviet Baltic states and ex-communist Poland.

Ukrainian President Petro Poroshenko described the entry of the trucks without Kiev's permission as a "flagrant violation of international law". But a senior security chief said Ukrainian forces would let them pass to avoid "provocations".

Kiev called on international allies to unite in "a decisive condemnation of illegal and aggressive actions" by Russia.

NATO also said Russia risked further international isolation. It has ruled out intervening militarily on behalf of Ukraine, which is not a member, and Europe has been reluctant to step up sanctions due to trade ties and its need for Russian gas.

Russia said it was not breaching international law and that Russian President Vladimir Putin had told German Chancellor Angela Merkel in a telephone call Moscow had been unable to wait any longer for Kiev's green light to help people in distress.

Merkel, who also spoke to Poroshenko, expressed her great concern, praising the Ukrainians for a "prudent" reaction and calling for a speedy ceasefire and shoring up of the frontier.

U.S. President Barack Obama spoke with Merkel and the two leaders agreed that sending the convoy into Ukraine was a "provocation" by Russia and called on Moscow to remove the convoy, the White House said in a statement.

They also expressed concern that the large numbers of Russian troops on the Ukraine border, the presence of Russian military in Ukraine, and Russian shelling of Ukrainian territory "represent a dangerous escalation," the White House said.

The UN Security Council met on Friday and British Ambassador Mark Lyall Grant told reporters after the meeting that no country had come to Moscow's defence, while many council members called the crossing of the aid convoy an "illegal and unilateral" action by Russia.

Mystery contents

Poroshenko said more than 100 trucks had crossed the border, and only some had been checked earlier by Ukrainian officials inside Russian territory. Other Ukrainian officials said only 34 or 35 of them had been properly checked.

Repeating earlier suspicions by Kiev that the aid cargo could be used to support the separatists, the foreign ministry said: "Neither the Ukrainian side nor the International Committee of the Red Cross knows the content of the trucks. This arouses special concern."

A Reuters witness said the white-painted trucks had crossed onto Ukrainian soil and headed towards the rebel stronghold of Luhansk escorted by a small number of separatist fighters.

The presence of the Russian trucks could force Ukrainian troops encircling Luhansk to rein in their offensive against the rebels there, because if they hit one of the Russian vehicles, that could give Moscow justification for a full-scale invasion.

Any lull in fighting that resulted would give a badly needed respite to the rebels in Luhansk, who have been facing defeat, and allow them to regroup.

The news that the convoy had finally crossed into Ukraine dominated Russian TV news and was certain to have further boosted Putin's standing at home.

But it equally cast a shadow over a meeting next Tuesday with Poroshenko and the European Union in the Belarusian capital of Minsk which has held out prospects of a breakthrough to end the confrontation.

Mikhail Denikin, chairman of the village council in Izvaryne, on the Ukrainian side of the border, stood by the road waving a large Russian flag as the trucks drove past.

"Big thanks to Russia. Our brothers did not forget us. We are brothers. That is the most important thing. We are Slavs, we are together," Denikin told Reuters Television.

A traffic police officer on the Russian side of the border, who had been escorting the aid convoy within Russian territory, said the entire convoy of about 260 trucks had passed into Ukraine. He said it was possible they would cross back into Russia on Friday evening after delivering their cargo.

"We consider this a direct invasion by Russia of Ukraine," Ukrainian state security chief Valentyn Nalivaychenko said in a statement to journalists. Asked whether Ukraine would use air strikes against the convoy, Nalivaychenko said: "Against them, no."

But Ukrainian authorities said the convoy would pass through an area where the rebels were firing so its security could not be guaranteed. Interfax news agency said later that the first trucks had reached rebel-held Luhansk.

Intense fighting

The largely Russian-speaking Donetsk and Luhansk regions both declared independence after a plebiscite deemed illegal by Kiev. The regions have seen intense fighting in recent weeks as rebels have been driven back into pockets.

Moscow, at odds with Ukraine since pro-Western protests overthrew a pro-Russian president in February, had earlier expressed impatience with delays with the convoy, which left the Moscow region around Aug. 13.

"We warn against any attempts to disrupt this purely humanitarian mission," the Russian foreign ministry said. "Responsibility for any possible consequences of provocations ... will lie, completely and entirely, with those who are prepared to further sacrifice human lives for the sake of their ambitions and geo-political ploys."

The International Committee of the Red Cross, which both Moscow and Kiev had agreed should supervise the convoy, said it was not escorting it "due to the volatile security situation".

The entry of the trucks ran counter to the arrangement agreed with the ICRC and was a clear violation of the border, said Sebastien Brabant, spokesman for the European Union's foreign policy chief, Catherine Ashton.

NATO went a step further. "These developments are even more worrying as they coincide with a major escalation in Russian military involvement in eastern Ukraine since mid-August, including the use of Russian forces," Secretary General Anders Fogh Rasmussen said.

NATO Military Commander General Philip Breedlove compared the convoy to Russian humanitarian and peacekeeping moves in Georgia, Moldova and Crimea. "We have seen how they proved to be deceptions," he said.

Kiev has been using troops, artillery and air power to try to quell a separatist rebellion that broke out soon after Russia annexed the Crimean peninsula from Ukraine in March.

The last few weeks have seen a string of rebel defeats in a conflict that has killed more than 2,000 people.

A Reuters cameraman said it had been possible to see inside some of the vehicles on Friday. The cargoes visible consisted of cardboard boxes with tinned food, pallets of bottled water, generators and other supplies.

Poroshenko said on Thursday he would call on Putin to rein in pro-Russian separatists when the two meet next week and told the Kremlin chief he had "a strong country, a strong army".

Merkel is scheduled to visit Kiev on Saturday to show her support for Poroshenko - but diplomats say she is also bearing a message that he should consider calling a ceasefire so as not to incur a backlash from Putin.

In Donetsk, pro-Russia separatist Denis Pushilin, guarded by men who identified themselves as Chechens, handed out aid -sugar, tea, canned beef and rice - and envelopes of money to three families in a state building in the city centre. The aid, collected in Russia by Russian citizens, was not connected to the aid crossing the border on Friday, Pushilin said.

"Hopefully soon we'll be able to start handing out aid to hundreds if not thousands of more families in need."

Rebels brought two destroyed Ukrainian armoured personnel carriers to Donetsk's central Lenin Square to display on Sunday, when rebels plan on parading prisoners of war through the streets of the city as a counterpoint to festivities planned in Kiev as part of Ukraine's Independence Day.

After four months of fighting in the industrial, Russian-speaking east of Ukraine, the area faces a humanitarian crisis, lacking supplies of food, medicine and clean water.