DAVID Cameron has given a cautious welcome to a ceasefire in Ukraine but insisted what was needed was a long-term peace plan, warning that the West would not accept any partition of the crisis-torn country.

The Prime Minister, denouncing Russia's actions in eastern Ukraine as wrong and indefensible, announced that Britain would head up a new rapid-reaction Nato "spearhead" force designed to deter Russian further aggression against former Soviet bloc states in eastern Europe.

On the final day of the alliance summit in South Wales, the Prime Minister said the UK would be the first member state to take charge of the 4000-strong force when it became operational at the end of 2015. The UK will provide the largest contingent with a 1000-strong battle group and headquarters for the force, which will be on standby to deploy troops within two to five days in the event of a new crisis.

Mr Cameron also confirmed the European Union would press ahead with sanctions against Russia, despite the agreement of a ceasefire with pro-Moscow rebels announced by Ukraine's President Petro Poroshenko.

Mr Cameron welcomed the truce but stressed Western sanctions would only be removed if "proper milestones" towards a lasting peace were reached. The PM hailed a "hard-fought" agreement by all 28 Nato members to reverse the trend of cuts in defence spending and move towards meeting the alliance's target of spending two per cent of GDP on defence over the coming decade, with a fifth of spending to go on major new equipment.

He sought to underline Britain's own commitment to alliance collective defence with an announcement that the Royal Navy's second new aircraft carrier - the Prince of Wales, due to be completed in 2017 - will enter service, enabling the UK to have a carrier available at all times. The creation of a force able to deploy thousands of troops in days is intended to reassure allies in eastern Europe, such as members Estonia, Latvia and Lithuania, who fear they could be the next targets of Soviet aggression.

Troops will be provided by member states on a rotation basis. It will have a headquarters in Poland and will be supported by the "pre-positioning" of supplies of in eastern Europe. Mr Cameron said Britain would also deploy 3,500 personnel on Nato exercises running to the end of 2015 intended to ensure an alliance presence while providing further financial assistance to Kiev.

"We will continue our efforts to support Ukraine, including by providing financial assistance to improve their command, control and communication capabilities," said the Prime Minister. "Nato members - particularly in central and eastern Europe - have been reassured that this is an alliance that will meet its treaty obligations to come to the defence of any member under threat.

"Our collective security is as strong as it has ever been," he added. US President Barack Obama said the agreement demonstrated Nato was "fully united" behind Ukraine's independence, territorial sovereignty and right to self-defence. "This commitment makes clear Nato will not be complacent," he said.

Mr Poroshenko, who attended the summit as a Nato partner, said the new ceasefire was based on a 12-point plan drawn from a phone call this week with Russian President Vladimir Putin. All hostages held by rebel groups are expected to be freed later today.

"Now it is very important that this ceasefire lasts long and we continue the political dialogue to bring peace and stability," said the Ukrainian president.