Developed jointly by the two NATO allies, Storm Shadow/SCALP is a 1,300-kilogramme (2,870 pounds) missile armed with conventional explosives, usually launched from aircraft such as the Royal Air Force's Eurofighter Typhoon or French Rafale.

Built by missile maker MBDA, its range of over 250 kilometres (155 miles) makes it the longest-range Western weapon supplied to Kyiv so far, capable of striking targets far into the country's Russian-occupied east.

French missile deliveries would "preserve the clarity and coherence of our doctrine, which is to allow Ukraine to defend its territory" from Russian invasion, President Emmanuel Macron said.

The subtext: French-supplied weapons should not be allowed to hit Russian territory, after Moscow's repeated warnings of reprisals.

Manufacturer MBDA says on its website that the SCALP is "designed to meet the demanding requirements of pre-planned attacks against high-value fixed or stationary targets such as hardened bunkers and key infrastructure".

It has been used in previous conflicts including in Iraq, Libya and Syria.

The missile uses inertial navigation, GPS and terrain referencing to chart a low-altitude course to its target to avoid detection.

It uses an infrared camera to match images of the target to a stored picture "to ensure a precision strike and minimal collateral damage," MBDA says.

The warhead can be programmed to detonate above the target (airburst), on impact or following penetration.

Russian-installed officials said last month that a British-supplied Storm Shadow had hit a bridge at Chongar, which links the annexed Crimean peninsula to southern Ukraine.

The bridge was "unusable" following the strike and would be closed for around 20 days, Moscow's governor for southern Ukrainian region Kherson, Vladimir Saldo, said at the time.

Russia claimed soon after Britain began delivering the missiles in May that it had already shot down a Storm Shadow.

But both sides in the conflict regularly claim to have destroyed the other's hyped high-tech weapons.

In recent months, Ukraine has claimed kills of Russia's Kinzhal hypersonic missiles and Moscow has highlighted successes against German-made Leopard tanks operated by Kyiv.

As with many Western arms supplied to Ukraine, France's stocks of the SCALP are not bottomless.

Trade magazine Defense et Securite Internationale has reported that Paris has "fewer than 400" of the missiles.