Britain believes that a European-led security guarantee to Ukraine could be provided with considerably fewer troops than the 100,000 to 150,000 called for by President Volodymyr Zelenskyy last week in the event of a peace agreement.

A European-led assurance force for Ukraine would not necessarily involve large numbers stationed on the frontline, defence sources indicated on Tuesday, and could therefore be in the region of a few tens of thousands strong – or even less.

The security package could make heavy use of intelligence, surveillance and long-range reconnaissance capabilities to monitor any Russian attempts to breach a line of control agreed in a ceasefire or final peace settlement.

An alternative scenario canvassed was that there would be no European troops stationed in Ukraine at all, and that the focus of the security effort would be on the long-range monitoring.

Though this would be obviously attractive to Russia, which said on Tuesday it will not accept any troops from a Nato country being stationed in Ukraine, it would be unacceptable to Kyiv, which fears a renewed attack from Moscow and wants as large a postwar security force as possible.

Last week Zelenskyy used an interview with the Guardian to call for a deterrence force of between 100,000 and 150,000 to be based in Ukraine, although that number would be almost impossible to achieve with just European troops.

Other British military sources said the most likely scenario would be that the UK could contribute a brigade or possibly two, to a multinational division that it would lead. A brigade typically comprises a few thousand troops.

Constraining any UK contribution is the small size of the British army – at 71,347 at the last count in October. Britain is also committed to providing a brigade to defend Estonia in the event of a crisis, and there could be further pressures across Europe if the US decides to withdraw some of its 100,000 troops in Europe.

On Monday, the Washington Post reported that the size of a European force under discussion could amount to about 30,000 and while UK sources would not confirm this figure while talks were under way with Europe they did indicate that a force of this magnitude was most likely envisaged.

In public, the UK emphasised again on Tuesday that it would be critical for the US to provide a backstop. Although British leaders did not specify what that would mean, it is most likely to refer to air support, given the US has ruled out deploying its own troops in Ukraine.

John Healey, the defence secretary, echoed language used by the prime minister, Keir Starmer, on Monday when he said that any European security force would require some degree of US support to deter further Russian aggression.

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“The European countries have to play a leading part in that guarantee, but require a backstop from the US because, in the end, it is only the US that can provide the deterrence to Putin that will prevent him attacking again,” Healey said.

Starmer is expected to lobby the US president, Donald Trump, directly on the topic when he flies over to Washington next week, raising the stakes for what could be a critical meeting in the efforts to find a peace settlement to end the Ukraine war.

Though Russia’s foreign minister, Sergei Lavrov, ruled out any Nato countries providing an assurance force to Ukraine on Tuesday, the US said last week that it wants the future security of Ukraine to be led by Europe.

The situation remains fluid and uncertain, and European leaders have been scrambling to work out what their countries are willing to offer to a possible postwar assurance force in an effort that has been promoted most enthusiastically by France.

The topic was a subject of discussion on Monday at a meeting of European leaders hosted in Paris by the French president, Emmanuel Macron.