Prior to the Russian invasion of Ukraine on 24 February, Nato’s current presence on its eastern flank is known to have served as a more scaled-back “tripwire” for any Russian aggression.
Mr Stoltenberg pointed out that Nato has 40,000 troops on its eastern borders, a number almost 10 times that of before the invasion.
He added: “What we see now is a new reality, a new normal for European security. Therefore, we have now asked our military commanders to provide options for what we call a reset, a longer-term adaptation of Nato.
“I expect that Nato leaders will make decisions on this when they meet in Madrid at the Nato summit in June.”
He described the “reset” as “to move from tripwire deterrence to something which is more about deterrence by denial or defence.”
Mr Stoltenberg added: “This is already in process. We have to ensure that we continue to be able, in a more dangerous world, to protect and defend all Nato allies.”
Last month, the UK pledged to double its troops in Eastern Europe and send a new deployment to Bulgaria.
In the interview, Mr Stoltenberg urged other countries to emulate Britain’s support for Ukraine.
On Saturday, Mr Johnson made a secretly-planned visit to the Ukrainian capital to discuss with Mr Zelensky the UK’s plans to offer Ukraine more military aid including anti-ship missiles and 120 armoured vehicles. On Friday, the UK announced a further £100 million worth of equipment, including anti-tank and anti-aircraft weaponry.
Mr Johnson said: “Ukraine has defied the odds and pushed back Russian forces from the gates of Kyiv, achieving the greatest feat of arms of the 21st century.”
In a joint television appearance with Mr Zelensky, he said: “I think that the Ukrainians have shown the courage of a lion, and you Volodymyr have given the roar of that lion.
“Having been here in Kyiv for just a few hours, I have no doubt at all that an independent sovereign Ukraine will rise again thanks above all to the heroism, the courage of the people of Ukraine.”
In a video address, Mr Zelensky said Mr Johnson has made history with pledges to provide more military and financial support to Ukraine.
In his speech, he said: “The leadership of the United Kingdom will be remembered forever in history for providing our country with the necessary assistance, especially defence, as well as leadership on sanctions policy.”