The Czech president, Petr Pavel, a decorated retired general who was previously Nato’s principal military adviser, has privately warned Ukraine’s leadership against the disaster of a rushed counteroffensive.

In recent meetings in Kyiv with Ukraine’s president, Volodymyr Zelenskiy, and prime minister, Denys Shmyhal, Pavel cautioned that they no longer had the element of surprise that aided successful assaults on the eastern city of Kharkiv and southern region of Kherson last year.

The war hero, who was chair of the Nato military committee until 2018 and decorated by the Czech and French governments for rescuing French troops besieged by the Serbs during the Bosnian war in 1993, said there was recognition in Kyiv that gaps remained in Ukraine’s capacity for a successful offensive against Russian forces.

He said Zelenskiy had asked him and his Slovak counterpart, Zuzana Čaputová, for their nations each to arm a Ukrainian mechanised brigade before the long-anticipated counter-assault.

“Apparently, they still have a feeling that they do not have everything to start successfully an operation,” Pavel said.

With preparations still being made, Pavel, who was elected in January, said he had appealed to Shmyhal during meetings last week not to be “pushed into a faster pace before they are fully prepared”.

“Because it might be a temptation to push them, for some, to demonstrate some results,” Pavel said in an interview during a visit to London for King Charles’s coronation. “It will be extremely harmful to Ukraine if this counteroffensive fails, because they will not have another chance, at least not this year.”

Ukraine would inevitably face “terrible losses” no matter the strength of its forces, he said, and it could not afford for the assault to fail. “Because it’s extremely demanding in terms of putting together personnel equipment, ammunition logistics, fuel financing. It will simply be one chance this year, so it has to be successful.”

In a wide-ranging interview – his first in the UK – Pavel also said:

• EU member states agreed this weekend to source ammunition for Ukraine from outside the bloc, including the UK and the US, despite initial objections from France, a decision he said would increase the scope for helping Ukraine in the next weeks and months.

• Europe did not have the capacity to produce the armaments it needs but it could buy it in and Zelenskiy had said he would provide qualified technicians for new ammunition factories if the EU defence industry fell short. “He said: ‘Yeah, we can do that.’”

• The EU should source ammunition for Ukraine from all over the world, including countries that might not want to admit to being involved in the conflict with Russia, or with whom European capitals might feel some diplomatic embarrassment in dealing with – “there are ways how we can do it”.

• Claims from the Kremlin that Ukraine had sought to assassinate Vladimir Putin through a drone strike in Moscow were “nonsense” given the defensive shield around the Russian capital, and could instead be a “pretext, for a bigger air attack on Ukraine”.

• China has made a “first step” that could help the west put diplomatic pressure on Putin by backing a UN resolution describing Russia as the “aggressor”, though Pavel said he remained doubtful that Beijing could be a trusted mediator. “Does China have a real interest to push hard on Russia and to make for Russia to make concessions? I don’t think so.”

• The west must be prepared for an outcome in the war short of all-out victory. “I think we should do anything at what is at our disposal to encourage Ukrainians and to support them to be successful. But internally, we should also be ready for other contingencies.”

Pavel said Zelenskiy’s main demand during their meeting on 28 April was for ammunition as he sought to build up his forces for this year’s spring push.

“Volodymyr Zelenskiy is a man who doesn’t have time to go around the problem and I indicated from the very first moment of our meeting that I’m also not a man of diplomatic language,” Pavel said. “So I encouraged him to go straight to the problem, and he said: ‘Well, OK, we need a lot of ammunition.’ And then he gave me a load of equipment that they need to build successfully and obligates for a counteroffensive.

“They are trying to set up up to nine mechanised brigades. And since he met me and Zuzana Čaputová on the same day, he asked us if we could take, each nation, one brigade, which is in effect something about 35 tanks and 100 armed personnel carriers (APCs) and ammunition.

“Now, it will be extremely difficult even though the Czech Republic has already delivered slightly less than 100 tanks, especially T72s, many of them modernised, about a similar number of APCs, artillery, rocket launchers and a lot of ammunition. Where we still have some capacity is air defence systems with a number of rockets, which will be very helpful to them.”

Pavel said Kyiv should not “underestimate the Russians because they have enough manpower, they still have enough equipment, even though it is older, probably in lower numbers, but still, they have a large amount of artillery barrels, they have a lot of ammunition.”

He added: “They are not so incapable in terms of defence. And of course, being in defence makes it easier for them because Ukraine will have suffered terrible losses, even if they are well prepared. So attacking an enemy like Russia will be difficult and Russians will not be caught by surprise for the second time.”

Full interview here.