Ukraine claims it has destroyed Russian military vehicles in the country's east, a day after a column was spotted moving across the border.

Ukraine's president, Petro Poroshenko, told David Cameron by phone that his country's armed forces had destroyed part of an armed convoy that the Guardian saw moving through a gap in a border fence on Thursday night.

There was no immediate proof, and it was impossible to establish if the Ukrainians had targeted the same convoy seen by the Guardian. The Russians categorically denied that any of their troops were even in Ukraine. But the claim marks a new escalation in the six-month confrontation over Ukraine and if verified would amount to the first confirmed military engagement between the two adversaries since the crisis began in the spring.

"The president informed [Cameron] that the information was trustworthy because the majority of those machines [Russian military vehicles] had been eliminated by the Ukrainian artillery at night," a statement from Poroshenko's office read.

A White House spokesman said last night: "The escalation in Russian activity designed to destabilise Ukraine in recent weeks is extremely dangerous and provocative."

The Guardian and Telegraph witnessed the convoy crossing through a gap in a wire demarcation fence on Thursday night, close to the village of Severny on the Ukrainian side. The convoy had waited by the side of the road several miles away until darkness fell, and then moved towards the border. As it crossed the fenced area armed men stood guard. It was impossible to verify the destination or ultimate fate of the convoy, or monitor how long it stayed on the other side of the border.

Lorries which Russia says are carrying humanitarian aid for Ukraine waiting near the border for approval to enter the country. Photograph: ITAR-TASS/Barcroft Media

Nato said it had observed the Russian incursion. "What we have seen last night is the continuation of what we have seen for some time," said the Nato secretary general, Anders Fogh Rasmussen. The reports drew swift international reaction. The Russian ambassador in London was summoned to the Foreign Office, and the foreign secretary, Philip Hammond, said he was "very alarmed" and that any incursion could have serious consequences.

Russia strongly denied any incursion. The FSB security service said it was in fact a mobile response team of border guards operating strictly within Russia's borders. A spokesperson for the FSB Border Guard Service in Rostov region told Russia Today: "When residents report about cross-border shooting and fighting in the frontier zone, these teams are immediately deployed to such areas to provide the safety of the Russian state border and Russian citizens, and also to prevent armed people from crossing into the territory of the Russian Federation."

President Vladimir Putin's spokesman Dmitry Peskov called the reports "fake". Russian defence ministry spokesman Igor Konashenkov said: "No Russian military column crossed the Russian-Ukrainian border by night or by day. It would be better if the Ukrainian artillery destroyed a phantom, rather than refugees or its own soldiers."

On Friday the Guardian again travelled to the site of the crossing to look at the area in daylight. A fence which demarcates the border and runs along the outer suburbs of the Russian border town of Donetsk is permeated by informal crossings and dirt tracks. Around the area where the Guardian saw the convoy the day before, a truck with Russian military plates was parked on the Russian side. It was not possible to linger as the area is a restricted zone, but military vehicles with no plates were seen coming from the direction of Ukraine, and a car carrying three men in fatigues sped in the direction of Ukraine. Several military-style vehicles with no licence plates were visible inside the Russian town of Donetsk.

Ukraine has long accused Russia of funnelling fighters and weapons across the porous border, including the Buk missile system that is believed to have brought down Malaysian Airlines flight MH17 over eastern Ukraine last month. Russian-backed separatist rebels are continuing to fight a rearguard action against Ukrainian armed forces in and around Donetsk in Ukraine, which has come under sustained attack in recent days.

A monitoring mission of the Organisation for Security and Co-operation in Europe (OSCE) is monitoring two official border points in the area. But the mission is not allowed to monitor other parts of the border, a limited mandate agreed after wrangling between the OSCE member states, which include Russia.

A Ukrainian army sapper holds a shell found by local residents just outside Donetsk. Photograph: Valentyn Ogirenko/Reuters

An OSCE representative in the region said there were frequent sightings of "young men in military-style dress" crossing the border at the official points, as well as some cases of wounded men returning from Ukraine. However, he said there had been no signs of military vehicles crossing the border. The OSCE has monitored the official border points 24 hours a day in the two weeks since its mission has started, but has not monitored unofficial crossings.

On Thursday there was further large-scale movement of military hardware along the road between the border and Kamensk-Shakhtinsky, where a caravan of more than 200 Russian aid trucks is awaiting onward shipment to eastern Ukraine to deliver humanitarian aid.

The Guardian saw at least 50 armoured personnel carriers and dozens of trucks and troop carrying vehicles, many emblazoned with "Peacekeeping Forces" and some flying the Russian flag, on the road. They appeared to turn off before reaching the immediate vicinity of the border. Most of the support vehicles had plates that identified them as belonging mainly to the Urals Military District, while others were from the North Caucasus region.

A military pointsman talks to the driver of a truck at the head of a column of military vehicles outside the Rostov region, near the border with Ukraine. Photograph: Maxim Shemetov/Reuters

Earlier in the day, what appeared to be parts of a BUK system were also seen in the area, passing by the roadside camp where the humanitarian convoy has stopped. The aid convoy set off from a Russian military base outside Moscow on Tuesday. Russia says it includes badly needed aid for residents trapped in Luhansk for almost two weeks without water and electricity. Officials from Russia's emergency ministry opened a number of trucks for journalists on Friday to show they were full of food and emergency supplies.

Dozens of Ukrainian border guards have arrived in the area and are in talks with Russian officials and the International Red Cross to allow the aid to cross into Ukraine.

Indeed, away from the fighting in Ukraine and the counter-claims about border crossings, there are diplomatic efforts to bring the two sides together. On Friday the Russian and Ukrainian presidential chiefs of staff met in Sochi. On Sunday foreign ministers will meet in Berlin to continue the dialogue.