Ukraine is facing a “forever war” and a slow erosion of territory unless Europe dramatically increases pressure on Russia, including by deploying troops and establishing a missile and drone shield on Nato territory to protect Ukraine from Russian attacks on its infrastructure, a former Nato secretary general has said.

Anders Fogh Rasmussen, who held the Nato post from 2009 to 2014 and was the prime minister of Denmark from 2001 to 2009, said in an interview with the Guardian that if countries such as Poland agreed to host such air defences, Russia would understand that an attack on Ukraine would be an attack on the whole of the Nato alliance.

He said: “We have to help the Ukrainian [people] protect themselves against Russian missiles and drones by building an air shield helping the Ukrainians shoot down Russian missiles and drones. Nato countries neighbouring Ukraine can be the location for a Nato-based air defence and missile system.”

Rasmussen also called for the deployment of a European protection force for Ukraine in advance of a ceasefire agreement. He said the “coalition of the willing” hoping to assemble such a force for when fighting ends had been reduced to a coalition of the waiting.

Anders Fogh Rasmussen said changes in strategy were needed as Putin had no incentive to engage in peace negotiations. Photograph: Olivier Matthys/EPA

“If we do not carry out major changes in strategy we will look into a forever war,” he said. “Putin has no incentive to engage in peace negotiations so long as he thinks he can win on the battlefield. Changes in speed and mindset are needed.”

Rasmussen, who has forged close links with the Ukrainian leadership, is touring European capitals, including London to meet the UK national security adviser, Jonathan Powell. The two men discussed whether the US could provide security guarantees to Ukraine, using language similar to that recently used by the US when it provided security guarantees to Qatar after Israel attacked its capital, Doha.

Iron-clad security guarantees would make it easier for Volodymyr Zelenskyy, Ukraine’s president, to sell a peace deal to his own people that involved the loss of Ukrainian territory, Rasmussen said.

He said it was necessary to make dramatic adjustments to force a change in Vladimir Putin’s mindset. Apart from the development of a Ukrainian missile and drone shield based in Europe, and the deployment of some European troops into Ukraine, he urged Ukraine to be given long-range missiles to hit more targets inside Russia.

He said it was still possible to reopen discussions on the supply of US Tomahawk cruise missiles to Ukraine, which Donald Trump has not backed, if Germany was willing to be “the first mover” and provide Ukraine with its own Taurus cruise missiles.

He said: “That would send a clear signal across the Atlantic and put pressure on the White House. It is a strong German interest to force Putin to engage in peace negotiations … Taurus is the means to do so.”

Trump’s approach to Ukraine is in limbo after the US president appeared to step back from supplying Tomahawk missiles and announced instead a second summit with Putin.

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Trump later cancelled that meeting, saying he did not believe the Russian president was serious about peace, and imposed economic sanctions against the largest Russian oil companies, Rosneft and Lukoil. He appears overall to have reverted to a position whereby the two sides have to be left to fight it out.

Rasmussen said Europe was still not waking up to the threat posed by Russia and urged the use of frozen Russian assets for Ukraine. “We need to defreeze the frozen €150bn [£132bn] Russian assets in Euroclear and use the assets as the basis for a loan to purchase weapons and hopefully start reconstruction,” he said.

He predicted European leaders would be able to overcome objections to this plan voiced at the last EU council of ministers, principally by Belgium, where Euroclear is based. The plan rests on the assumption that neither Ukraine nor the EU would need to repay the loan but could instead draw on a proposed Russian payment of war reparations to Ukraine.

Belgium, however, is saying there is no guarantee reparations will be paid as a part of a ceasefire and it may require all EU states to provide guarantees to Belgium about the loan’s repayment.