Anders Fogh Rasmussen and Jaap de Hoop Scheffer warn US president-elect against deal with Putin that would cede Crimea.

Two former Nato chiefs have called for an extraordinary summit soon after Donald Trump’s inauguration to reassure traditional allies that the US will still come to their defence.

Former Nato secretary general Anders Fogh Rasmussen and his predecessor Jaap de Hoop Scheffer also warned the US president-elect against making a hasty deal with Vladimir Putin that would cede Crimea and eastern Ukraine as a Russian sphere of influence.

They argue it would set a precedent for further expansionism in Russia’s “near-abroad”.

“If we accept the annexation of Crimea we will have given up on the rule-based order and it would have consequences elsewhere in the world,” Rasmussen told reporters in a call organised by the Atlantic Council thinktank.

During the election campaign, Trump said the people of Crimea appeared to want to live under Russian rule and said he would look at whether the US would recognise Moscow’s 2014 annexation of the peninsular. He also told the New York Times that allies would have to reimburse Washington for their protection or be told: “Congratulations, you will be defending yourself.”

Scheffer raised fears that Trump would turn campaign rhetoric into administration policy in a grand bargain with Putin, ceding Crimea and eastern Ukraine to Moscow, in return for non-interference in the Baltic states.

“In accepting the annexation of Crimea, it would be the first time since the second world war that borders have been changed by sheer force,” Scheffer said. “Such a deal would be considered by Russia and President Putin as a political alibi to expand his sphere of influence in what he qualifies as his ‘near abroad’. I think it would set a very bad precedent and might create in the Kremlin the wrong impression that if you wait long enough, Nato and the European Union and the Americans are finally giving in.”

On his farewell tour of Europe, Barack Obama has offered reassurance to nervous allies that Trump is committed to Nato despite disparaging remarks about the pact.

President Obama, who is meeting foreign allies in Berlin on Thursday, said “one of the most important functions I can serve at this stage, during this trip, is to let them know that there is no weakening of resolve when it comes to America’s commitment to maintaining a strong and robust Nato relationship, and a recognition that those alliances aren’t just good for Europe, they’re good for the United States, and they’re vital for the world.”

However, the former Nato chiefs said that such reassurances would be more effective if they came from Trump himself.

“I think it’s important to organise a Nato summit very soon after Mr Trump’s inauguration as the new American president,” Rasmussen said. He added that at such a summit the new president would reaffirm US commitment to defend all Nato allies, and those allies would promise to do more to honour existing pledges to spend 2% of their national income on defence. The alliance should also commit itself once more to supporting Ukraine independence and sovereignty.

Scheffer said Trump appeared from his campaign to be a man “who does not like alliances”, adding that he should act fast to reverse that impression.

“It is very important to come out for Nato as strongly as possible in the form of a summit,” Scheffer said, though both he and Rasmussen said such a meeting should be left until Trump appointees for top cabinet posts had been confirmed by the Senate.

A lack of clarity, Rasmussen said, “could lead to miscalculation by potential aggressors”.

“I’m concerned that President Putin might be emboldened already,” he added.