British troops to make up 25% of 'spearhead' unit initially based in eastern Europe to ease fears of Baltic member states

The UK is to contribute a quarter of the troops in a new 4,000-strong Nato rapid reaction force for initial deployment in the Baltics, easing anxiety among member states near Russia's border that the Kremlin could repeat its aggression in Ukraine elsewhere in the region.

The announcement will come against the backdrop of peace talks involving Russia and Ukraine in Minsk, the Belarusian capital, aimed at achieving a ceasefire to bring an end to the months of fighting between Ukrainian forces and pro-Russia separatists.

"We must be able to act more swiftly," the British prime minister, David Cameron, told Nato leaders at the beginning of a two-day summit in south Wales. "I hope that today we can agree a multinational spearhead force deployable anywhere in the world in just two to five days."

The UK contribution is intended to signal that the British government, in spite of the parliamentary vote against intervention in Syria last year and the drawdown from Afghanistan, is not entering an isolationist phase.

Government sources said the UK would contribute a battle group and a command HQ to the new force, which was to be formally unveiled on Friday at the Celtic Manor golf resort near Cardiff.

The unit would be able to deploy a "spearhead group" within 48 hours to reinforce the Baltic states in the event of a Russian invasion, followed by reinforcements. But Nato officials said it could also be used anywhere else in the world.

The new force is aimed at meeting criticism that Nato is too slow and unwieldy and increasingly irrelevant. Nato hopes the main components of the force will be in place by the end of the year.

Cameron, speaking at the start of the Friday session alongside President Barack Obama and other leaders, said Nato must increase [its] capacity.

The 1,000-member British contribution is to try to speed up the formation of the rapid reaction force, which will be staffed on a rotational basis with troops from other Nato countries.

Cameron also spoke about a British contribution of 3,500 troops – but this was a reference not to the spearhead group but to British forces already engaged in exercises in the Baltics and elsewhere in eastern Europe.

There are 1,500 British troops involved in exercises in Baltic states, and the extra 2,000 is aimed at sending a warning to Russia against attempting to repeat tactics used in Ukraine in the Baltic states, which also have large ethnic Russian populations.

The 3,500 figure includes air as well as land forces. Britain is already committed to a major exercise being held in Poland in October.

Nato is reluctant to break a 1997 treaty with Russia that forbids placing permanent bases in the Baltic states and is getting round this by holding permanent exercises, which have largely the same impact of reassuring the Baltic governments. Air bases in the Baltics are being modernised and heavy equipment being put in place in readiness.

Unlike Ukraine, the Baltic states are members of Nato and any action against them would theoretically draw in other member states.

The Nato secretary-general Anders Fogh Rasmussen said: "Nato remains ready for the defence of all allies against any threat. We have to ensure we have the right forces and the right equipment in place for as long as required … Nato always rises to every challenge, we stand ready to act together."

Nato is avoiding direct military involvement in Ukraine and is instead looking to the US and the EU to maintain pressure through sanctions.

The UK foreign secretary, Philip Hammond, predicted that the EU would impose new sanctions against Russia in spite of the proposed ceasefire between Ukraine and pro-Russia separatists due to start on Friday.

Hammond echoed Rasmussen in expressing scepticism about whether the ceasefire would be implemented and whether it would hold. "If there is a ceasefire, if it is signed and if it is then implemented, we can then look at lifting sanctions. But there is a great degree of scepticism about whether this action will materialise, whether the ceasefire will be real," he told BBC News.

"We can always take the sanctions off afterwards. I don't think we want to be distracted from our determination to impose further sanctions in response to Russia's major military adventure into Ukraine by these noises off about a possible ceasefire."