By Andrew MacAskill and William James

LONDON (Reuters) -Britain on Thursday became the first country to start supplying Ukraine with long-range cruise missiles, which will allow Ukrainian forces to hit Russian troops and supply dumps deep behind the front lines.

Ukraine has been asking for months for long-range missiles, but support provided by Britain and other allies such as the United States has previously been limited to shorter range weapons.

Highlighting what he called the deliberate targeting of civilians, Britain's Defence Minister Ben Wallace told parliament: "Russia must recognise that its actions alone have led to such systems being provided."

Wallace said Britain was supplying the Storm Shadow missiles to Ukraine so they could be used within its territory, implying he had received assurances from Ukraine that they would not be used to target inside Russia.

The missiles "are now going into, or are in, the country itself", he said.

The Kremlin earlier said that if Britain provided these missiles it would require "an adequate response from our military".

Russia has launched a flurry of long-range missile strikes against Ukraine in recent days. It has previously said such attacks were designed to degrade Kyiv's ability to fight and that it does not deliberately target civilians.

Ukraine is expected to unleash a counteroffensive soon after six months of keeping its forces on the defensive. Russia mounted a huge winter offensive that failed to capture significant territory.

TANKS AND PILOT TRAINING

Storm Shadows, manufactured by European missile maker MBDA, are air-launched long-range missiles, designed for attacks against high value targets such as hardened bunkers and have a range of more than 250 km (155 miles).

Sidharth Kaushal, a research fellow in sea power at the London-based Royal United Services Institute, said Britain's decision to supply Storm Shadows was significant for Ukrainian forces in two ways.

Kaushal said it would put Russian ammunition depots in range again after Russian troops adapted to the introduction of U.S.-supplied High Mobility Artillery Rocket System (HIMARS) last year by moving them out of range of about 70 km (45 miles).

Secondly, he said Storm Shadows could be used to target Russian ships in the Crimean port of Sevastopol, which he said was significant because the Russian navy has been involved in launching strikes across Ukraine.

After the United States, Britain has been the second-largest supplier of military aid to Ukraine contributing 2.3 billion pounds ($2.9 billion) worth of support last year.

Although this is well below what the United States has provided, Britain has in the past been the first country to supply more sophisticated weapons to Ukraine.

Britain sent the first shoulder-launched anti-air and anti-tank weapons to Ukraine in the run-up to the invasion and in February announced it would be the first country to begin training Ukrainian pilots on NATO fighter jets.

In January, Britain said it would send 14 of its battle tanks to Ukraine, a pledge that was followed by other nations including the United States and Germany.

Wallace said the British supply of Storm Shadows would be enough to meet Ukraine's current demand.

He also referred to a possible second wave of missile supplies by a British-led group of European countries, which last week asked companies for expressions of interest to supply Ukraine with missiles with a range of up to 300 km (190 miles).

Wallace said one of the main challenges supplying the weapons was finding a way to incorporate a British-French designed weapon onto Russian-designed aircraft used by Ukrainian forces.

To date, the longest-range weapons that the United States has said it would provide is the Ground Launched Small Diameter Bomb (GLSDB), which has a range of about 150 km.

The U.S. has so far declined to supply Ukraine with the 297-km range Army Tactical Missile System (ATACMS) missiles amid concerns that Ukraine could use them to strike inside Russia's internationally recognised borders.

Britain said it always assessed the risk of provocation or escalation when deciding what to send to Ukraine.

(Reporting by William James and Andrew MacAskill; Editing by Kate Holton, Angus MacSwan and Alex Richardson)