Both Ukrainian military and pro-Russia rebels have agreed to a ceasefire under terms proposed by Vladimir Putin

Nato leaders cautiously welcomed an apparent breakthrough in the five-month Ukrainian conflict after the country's president, Petro Poroshenko, and one of the leading pro-Russia separatist leaders agreed to order a ceasefire on Friday.

But Poroshenko, who expressed cautious optimism about the truce, caught Nato officials off-guard with the disclosure that, while Nato was not arming Ukraine, at least one country, which he did not name, was providing Kiev with high-precision weapons.

The ceasefire, part of a seven-point peace plan proposed by the Russian president, Vladimir Putin, on Wednesday, would freeze forces in their positions at around midday on Friday.

Meanwhile, events on the ground remained volatile on Thursday, with a column of rebel armour spotted in the direction of Mariupol and reports of Ukrainian forces shelling the outskirts of Donetsk. In spite of the proposed truce, the US and the UK remain sceptical about Putin's intentions and urged more diplomatic and economic pressure, including the threat of increased sanctions. As part of that, the EU is expected to go ahead on Friday with new sanctions against Russia.

Poroshenko made the ceasefire announcement at the Nato summit in Newport where he met David Cameron with US president Barack Obama, German chancellor Angela Merkel, French president François Hollande and Italian prime minister Matteo Renzi.

At a press conference with the Nato secretary general Anders Fogh Rasmussen, Poroshenko said he hoped the ceasefire would happen, but he remained suspicious because Putin had proposed it.

Rasmussen said he had seen similar moves before by Russia that had been smokescreens, but he would welcome a serious political settlement.

Poroshenko said that provided a peace meeting in Minsk planned for Friday between Russia, Ukraine and the European security organisation the OSCE goes ahead as planned, he would call a halt to Ukrainian military attempts to regain territory held by the separatists. "I will call on the general staff to set up a bilateral ceasefire," he said.

Alexander Zakharchenko, the separatist leader, said he would order a ceasefire an hour later. The rebels are proposing the creation of "security zones" that would be policed by representatives of the OSCE and the establishment of a corridor for refugees.

Reaching a lasting peace agreement will be difficult. Ukrainian authorities are unlikely to accept any settlement that involves recognising rebel control over parts of Ukrainian territory, but are aware that Kiev's forces have suffered such losses in the past fortnight that they cannot go on fighting.

The White House expressed support for Poroshenko's "efforts to achieve a peaceful resolution to the conflict", and pinned the blame for the conflict on Putin, condemning Russia's "flagrant violation of Ukraine's sovereignty".

Nato has ruled out intervening in Ukraine with troops or equipment and pressure from the West has come mainly through sanctions.

Poroshenko caught Nato officials off-guard with the disclosure. He did not name the country involved but only a relatively small number of countries, such as the US, Poland or even Britain, have both the necessary equipment and the sympathy for the Ukrainian cause.

Such weapons are desperately needed by Ukraine because it is confronting an estimated 100 tanks and it does not have the kind of anti-tank weapons that can be fired at distance.

The number of Russian troops were said by Ukraine to be in the thousands rather than just 1,000, suggesting an escalation rather than de-escalation. Such an assessment by Poroshenko casts serious doubts over the prospect of a deal in Minsk.

Nato, at the end of the two-day summit, is to issue a strongly-worded document condemning Russian actions in the Ukraine, declaring Moscow to be in breach of international agreements. There have been disagreements over the wording with the US, Britain and Poland seeking tough language and Germany seeking to water it down.

European Union ambassadors meeting in Brussels agreed that restrictions imposed on Russian state-owned banks will be extended to state-owned defence and energy firms. The US and Britain pushed for the sanctions to go ahead in spite of the ceasefire but other countries were more hesitant.

The sanctions come after France, under pressure for going ahead with the export of Mistral helicopter carriers to Russia, finally caved in and said it will delay the contract because of events in Ukraine.

Hollande said delivery of the first ship, which had been scheduled for October, is dependent on the ceasefire holding and a political settlement being reached.

Some European foreign ministers sounded more upbeat about the ceasefire than the White House. A German spokesman said that the contacts between Kiev and Moscow appeared more solid now than at any time over the last five months.