Defence Secretary Ben Wallace told journalists: "On the process of jets, I've been pretty clear. One thing I've learned over the last year is don't rule anything in, don't rule anything out."

Kyiv has requested American-made F-16 warplanes to help repel the Russian invasion.

The United States has ruled out any deliveries of F-16s to Ukraine for now, but other partners including Poland have shown themselves more open to the idea.

"I'm very open to examining all sorts of systems, and not just jets, to give Ukraine that assistance," Wallace said.

The minister said that "these things don't always happen overnight. But I can say, we're not putting the Ukrainians at risk."

His comments came after Downing Street had appeared to rule out sending its combat planes.

"The UK's Typhoon and F-35 fighter jets are extremely sophisticated and take months to learn how to fly," the prime minister's official spokesman said Tuesday.

"Given that, we believe it is not practical to send those jets into Ukraine."

Britain's government said in January it was aiming to send tanks to Ukraine at the end of March, after becoming the first Western ally to promise heavy assault vehicles, with a plan to send 14 Challenger 2 tanks.

Wallace said that Kyiv's immediate need was for weaponry to allow military formations on the ground to push back Russian troops, and it was "easy to get carried away".

Supplying fighter jets would not be an overnight game changer due to the need for complex training, he added.

"You know, even if tomorrow morning we announced we were going to put them in fast jets that would take months," he said, as Ukrainians would face "suddenly having to learn to pilot" them.

"So there is no magic wand in this horrendous conflict," Wallace said, speaking at a press conference in Portsmouth in southern England with his Australian counterpart Richard Marles.