David Cameron has expressed “grave concerns” at reports that Russian military vehicles may have crossed into Ukraine and urged Moscow to find a peaceful resolution to the conflict.
In a telephone call with Ukrainian president Petro Poroshenko, the Prime Minister also warned a controversial aid convoy sent to the region by Russia must not be used “as a pretext for further provocation”.
Russia has denied that its troops entered its neighbour’s territory, insisting a convoy including armoured personnel carriers reported to have crossed the frontier were border guards who remained at all times on home soil.
Following the call, a Downing Street spokeswoman said: “They discussed the Russian humanitarian convoy on the Ukrainian border and the Prime Minister expressed grave concerns at reports of Russian military vehicles crossing the border.
“They agreed that humanitarian aid does need to reach those in the East but this should be delivered through the International Committee of the Red Cross.
“Russia should be co-operating fully with the ICRC and must not use the aid convoy as a pretext for further provocation.
“Both leaders also agreed on finding a political solution to de-escalate the situation and the Prime Minister welcomed President Poroshenko’s readiness to engage with Russia and other partners to achieve this. Russia needs to demonstrate the same willingness to find a peaceful resolution to the conflict.”
The UK has made £1 million available to support the humanitarian work, Mr Cameron told the president.
Ukrainian officials were earlier allowed to inspect the aid convoy, which Russia despatched for distribution around the rebel-held city of Luhansk.
In Moscow, a spokesman for the Russian Defence Ministry denied reports that Ukraine had destroyed Russian military vehicles.
Russian news wires quoted General Major Igor Konashenkov as saying that no Russian military convoy had crossed the border as Ukraine claimed.
Earlier, Russia said Russian forces were patrolling the border region.
Nato secretary-general Anders Fogh Rasmussen, however, confirmed that the alliance had observed a Russian “incursion” into Ukraine.
“What we have seen last night is the continuation of what we have seen for some time,” he said during a visit to Copenhagen.
Foreign Secretary Philip Hammond said the situation was “potentially very dangerous” as he arrived to discuss the situation with fellow EU foreign ministers.
Tensions between the West and Russian president Vladimir Putin have escalated following the apparent shooting down of Malaysia Airlines flight MH17 by Moscow-backed separatists.
After the meeting, EU foreign ministers urged Russia “to put an immediate stop to any form of border hostilities, in particular to the flow of arms, military advisers and armed personnel into the conflict region, and to withdraw its forces from the border”.
They said they remained “ready to consider further steps” in addition to the already-imposed sanctions on Russia and urged other countries not to seek to exploit a Moscow-imposed ban on some European food imports.