Ukraine needs more time before beginning a highly anticipated counter-offensive against Russian forces, President Volodymyr Zelensky has told the BBC, as the UK prepares to send Storm Shadow missiles to help Kyiv.

Britain's decision will make it the first country to provide longer-range missiles to Kyiv, which has been training a new contingent of forces and stockpiling Western-supplied munitions and hardware.

Analysts say these steps will be key to reclaiming territory captured by Russia, although the timing of the counter-offensive remains a question.

"Mentally we're ready...," Zelensky said in an interview with the BBC published Thursday. "In terms of equipment, not everything has arrived yet.

"With (what we have) we can go forward and be successful. But we'd lose a lot of people. I think that's unacceptable. So we need to wait. We still need a bit more time," he said.

But the head of Russia's Wagner private military company, Yevgeny Prigozhin, accused Zelensky of being "dishonest" in the interview.

Ukraine's counter-offensive "is in full swing", Prigozhin said.

Kyiv has spent months preparing to claw back ground in the eastern Donetsk and Lugansk regions, as well as the Kherson and Zaporizhzhia regions in the south.

- South Africa spat -

Separately on Thursday, the US envoy to South Africa accused the country of having covertly provided arms to Russia, despite its professed neutrality in the Ukraine war.

Ambassador Reuben Brigety said the United States was "confident" weapons and ammunition had been loaded onto a Russian freighter that docked at a Cape Town naval base in December.

"The arming of the Russians is extremely serious, and we do not consider this issue to be resolved, and we would like SA to (start) practising its non-alignment policy," Brigety was quoted as saying.

South Africa hit back against the charge, with presidential spokesperson Vincent Magwenya calling the envoy's statement "a counter-productive public posture" undermining "the spirit of cooperation" between the countries.

"While no evidence has been provided to date to support these allegations, the government has undertaken to institute an independent enquiry to be led by a retired judge," Magwenya added.

- Western arms -

The Storm Shadow missiles pledged by the UK have been used by British and French forces in the Gulf, Iraq and Libya and can be operated in extreme conditions.

"The donation of these weapons systems gives Ukraine the best chance to defend themselves against Russia's continued brutality," UK Defence Secretary Ben Wallace said.

His Ukrainian counterpart Oleksiy Reznikov said last month that Kyiv's preparations were "coming to an end" and his forces were ready "in a global sense".

But he also said that Abrams tanks promised by the United States would not be able to take part in the offensive because they would not arrive in Ukraine until the end of this year.

Ukraine has, however, received hundreds of other tanks, aircraft, munitions and other arms from its Western allies.

Since the start of Russia's invasion, Kyiv has received more than $150 billion in aid, including $65 billion in military assistance, according to a count by the German defence ministry.

Ukraine is counting on the success of its planned counter-offensive, as that could determine how much aid the West will be willing to donate in the future.

- Counter-attacks in Bakhmut -

Some voices are calling for peace talks with Russia, but in the BBC interview Zelensky rejected the possibility of land concessions.

"Why should any country of the world give (Russian President Vladimir) Putin its territory?" he said.

Russia was "counting on" a "frozen conflict", he warned.

But Western sanctions are having an effect on Russia's defence industry, he stressed. "We already see that they've reduced shelling per day in some areas."

A senior Ukrainian military official said earlier this week that Russian forces had dropped back from some areas near Bakhmut after limited counter-attacks by Kyiv's forces around the eastern city.

Prigozhin, whose forces are on the front line of the battle for Bakhmut, acknowledged that some Ukrainian units were successfully breaking through in some areas.

"The Ukrainian army's plan is in action... All the units which have been trained, which have received weapons, tanks and everything they need are already fully engaged," he said.

Russia's defence ministry, however, refuted Prigozhin's claims late Thursday, saying reports of breakthroughs around Bakhmut "do not correspond to reality".

Prigozhin is involved in a long-running dispute with Russian military chiefs over ammunition supplies for his fighters, whom he has threatened to pull out of Bakhmut.