Downing Street has insisted Britain is playing an "active role" in Europe's response to Russian aggression in Ukraine, after David Cameron faced accusations that he has become a "diplomatic irrelevance" as France and Germany spearhead a diplomatic effort to end the conflict.

French president Francois Hollande and German chancellor Angela Merkel were meeting Russian president Vladimir Putin in Moscow today to discuss a peace plan aimed at ending the fighting between Ukrainian government forces and Russian-backed separatist rebels.

The move has sparked fears of a division within the western alliance - with Britain and the United States apparently sidelined by the Franco-German initiative.

Britain's former top Nato commander General Sir Richard Shirreff said the UK should be playing a major role in the efforts to resolve the crisis amid fears that it could lead to an all-out European war.

He told BBC Radio 4's Today programme: "This is the most serious crisis to have faced Europe, arguably, since the Soviet invasion of Czechoslovakia in 1968. There is a threat of total war.

"The UK is a major Nato member, it is a major EU member, it is a member of the UN Security Council, and it is unfortunate that the weight that the British Prime Minister could bring to efforts to resolve this crisis appear to be absent."

Shadow foreign secretary Douglas Alexander said Gen Sir Richard's comments were a "serious indictment" of Mr Cameron's foreign policy.

"This isn't the first time that his Government has been one step behind our allies in trying to negotiate a diplomatic resolution to the crisis in Ukraine," he said.

"This warning that he is a 'bit player' in foreign policy, is yet more confirmation that David Cameron is weakening Britain's influence abroad, at the time when it is most needed."

The Franco-German initiative comes as US secretary of state John Kerry signalled that US president Barack Obama was considering supplying arms to Kiev government forces - a move opposed by Paris and Berlin.

Downing Street has sought to play down suggestions of a diplomatic rift, insisting that the visit was part of the so-called Normandy Group process, which began during last year's D-Day commemorations, in which France and Germany have taken the lead.

A No 10 spokeswoman said: "I don't think that anyone sat around the council table in Brussels at the extraordinary Foreign Affairs Council last week thought that Britain was playing an irrelevant role when we were pushing hard for a unanimous, strong and united message on the situation in Russia and Ukraine and leading the way on further work on sanctions."

The spokeswoman said that Britain was involved in discussions last year on the establishment of the Normandy Group and agreed the format, which has seen France, Germany, Russia and Ukraine engage in discussions at foreign minister and leader level on several occasions over the past eight months.

"We have been engaged in discussions about the work that is ongoing but it was always clear that it would be a French and German visit," said the spokeswoman.

"We've been comfortable with that process. We've worked very closely with Germany on the situation in Ukraine and it was something the Prime Minister and Chancellor spoke about at length when they met in January."

Mr Cameron had played a "leading role" in bringing about agreement on EU sanctions to put economic pressure on Russia, she said.

"It is very clear to people where our position on Russia and Ukraine has been. We have played an active role," said the spokeswoman.

"We are supportive of efforts to find a diplomatic solution to the crisis. We've been clear throughout that that is what we want to see, and we've been working closely with the Germans, French, EU partners and the US throughout. Let's see how talks go in Moscow today.

"We want a diplomatic solution. That solution will be found on the ground in eastern Ukraine and we will judge what progress we are making by actions there."

Meanwhile, Ukraine's ambassador-at-large Dmytro Kuleba has stepped up appeals for western arms to match the firepower of the rebels, who are being supplied with state-of-the-art weaponry by Moscow.

He told the Today programme: "We do need support, we need defensive weapons. Because Russia is supplying terrorists on a daily basis with new tanks, with new military equipment and we are only using our own resources - and, to be honest, they are quite outdated, because we are not investing in our military.

"There is already a conflict in Europe. We are fighting it, we are defending our country. It is up to Europe to decide whether to stand beside us."

Western diplomats said that Mrs Merkel and Mr Hollande - who yesterday met for talks in Kiev with Ukrainian president Petro Poroshenko - were going to Moscow with a repackaged version of a peace plan originally drawn up by Mr Putin.

The new version is reported to have dropped what were said to have been the most objectionable elements of the Putin plan in order to meet the concerns of the Ukrainians and the Europeans.

Mr Kerry - who was also in Kiev yesterday although he did not meet the French and German leaders - said that Washington had been informed of the Russian proposal, although he did not have all the details.

He told a news conference with Ukrainian prime minister Arseniy Yatsenyuk that Mr Obama was "reviewing all his options" in the wake of the renewed fighting which has flared up over the past two weeks.

"Among those options obviously is the possibility of providing defensive systems to Ukraine," he said. "We are not interested in a proxy war. Our objective is to change Russia's behaviour."

In Brussels, Nato secretary-general Jens Stoltenberg announced the alliance was increasing the size of its response force from 13,000 to 30,000 troops in the wake of events in Ukraine. The alliance has also moved to form a quick-reaction force of 5,000 ground troops.

Gen Sir Richard said that the alliance needed to send a clear message to Moscow that any move to widen the conflict would be met with a firm response.

"From a Nato perspective, the most important thing is to hold up a large stop sign to President Putin to ensure that there is no overspill from the Ukraine crisis," he said.

"What we need to see is a building up again of a credible conventional deterrence to send a very strong message to Russia and Mr Putin that if there is any hint of stepping across into Nato territory he will regret it."

The Foreign Office called on Russia to honour the commitments it made in ceasefire talks held last September in the Belarus capital, Minsk.

"We support diplomatic efforts that aim to bring about a peaceful resolution of the crisis in eastern Ukraine and secure Ukraine's territorial integrity in the face of Russian-backed aggression," a spokesman said.

"UK has played a leading role in the EU and Nato. We have consistently argued for a strong response to Russia's actions, pressing for strong sanctions in the EU and increasing our contribution to Nato assurance measures in Eastern Europe. We have been supportive of the Normandy process since it began last year."