The United Kingdom has delivered multiple "Storm Shadow" cruise missiles to Ukraine, giving the nation a new long-range strike capability in advance of a highly anticipated counteroffensive against Russian forces, multiple senior Western officials told CNN.

UK Defense Secretary Ben Wallace, calling the donation Ukraine's "best chance to defend themselves against Russia's continued brutality," confirmed the transaction on Thursday after CNN exclusively reported the deal.

The Storm Shadow is a long-range cruise missile with stealth capabilities, jointly developed by the UK and France, which is typically launched from the air. With a firing range in excess of 250km, or 155 miles, it is just short of the 185-mile range capability of the US-made surface-to-surface Army Tactical Missile Systems, or ATACMS, that Ukraine has long asked for.

Critically, the Storm Shadow has the range to strike deep into Russian-held territory in Eastern Ukraine. A Western official told CNN that the UK has received assurances from the Ukrainian government that these missiles will be used only within Ukrainian sovereign territory and not inside Russia. UK officials have made frequent public statements identifying Crimea as Ukrainian sovereign territory, describing it as "illegally annexed."

The missile is "a real game changer from a range perspective," a senior US military official told CNN and gives Kyiv a capability it has been requesting since the outset of the war. As CNN has reported, Ukraine's current maximum range on US-provided weapons is around 49 miles.

The deployment of the missiles comes as Ukrainian forces prepare to launch a counteroffensive intended to retake Kremlin-held territory in the eastern and the southern parts of the country.

Ukrainian President Volodymyr Zelensky said his country still needs "a bit more time" before it launches the counteroffensive, in order to allow some more of the promised Western military aid to arrive in country.

"With [what we have] we can go forward and be successful," Zelensky told European public service broadcasters in an interview published on Thursday. "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 added. Among the supplies Ukraine is still waiting for are armored vehicles -- including tanks -- which Zelensky said were "arriving in batches."

This is not the first time Britain has gone further than the US in the weaponry it has been prepared to send to Ukraine. It was the first ally to announce it was sending modern Western tanks to Ukraine, in January pledging 14 Challenge 2 tanks before the US announced it would contribute M-1 Abrams tanks shortly after.

Earlier this year, British Prime Minister Rishi Sunak signaled that Britain was considering sending long-range weapons.

"We must help Ukraine to shield its cities from Russian bombs and Iranian drones," Sunak said at the Munich Security Conference on February 18. "And that's why the UK will be the first country to provide Ukraine with longer-range weapons."

And earlier this month the British government issued a procurement notice through the International Fund for Ukraine. The notice said the UK was inviting expressions of interest for buying "long-range strike" rockets or missiles by May 4, and potential suppliers would be contacted after a month. The notice stipulated "missiles or rockets with a range 100-300km; land, sea or air launch. Payload 20-490kg."

US officials have repeatedly emphasized that they will continue supporting Ukraine for "as long as it takes", and while tens of billions of dollars' worth of equipment have been provided, the embattled country has continued asking for more to defeat the Russian military, including longer-range missiles such as ATACMS.

However, the US has been cautious over the last year in providing weapons to Ukraine that could help them strike within Russian territory. Under Secretary of Defense for Policy Colin Kahl told reporters in August that it is the US assessment Ukraine does not "currently require ATACMS to service targets that are directly relevant to the current fight."

According to MBDA Missile Systems, the European company which manufactures the missile, the Storm Shadow is a "deep strike weapon" capable of "being operated day and night in all weathers," that features an advanced navigation system to ensure accuracy.

"After launch, the weapon descends to terrain hugging altitude to avoid detection," MBDA's website states. "On approaching the target, its onboard infrared seeker matches the target image with the stored picture to ensure a precision strike and minimal collateral damage."

This story has been updated with additional details.