Russia fired over 300 Iranian-made drones at Ukraine in May, the UK said.
The goal was to get Ukraine to waste its valuable stock of air defense missiles, the UK MOD said.
But the efforts failed, with 90% shot down without Ukraine needing to use advanced missiles, it said.
Russia launched more than 300 Iranian-made drones at Ukraine throughout May, in an attempt to get the country to use up its stockpile of advanced air defense missiles, but the efforts didn't work, according to UK intelligence.
In an intelligence update on Monday, the UK Ministry of Defence labeled the campaign Russia's "most intense use" of Iranian Shahed uncrewed aerial vehicles to date, and said Russia was likely firing the drones in "an attempt to force Ukraine to fire stocks of valuable, advanced air defence missiles."
But it said that Russia was "unlikely to have been notably successful: Ukraine has neutralized at least 90% of the incoming OWA-UAVs mostly using its older and cheaper air defence weapons and with electronic jamming."
Shahed drones are estimated to cost upwards of $21,000 — relatively cheap compared to the cost of missiles, which Russia has also been firing at Ukraine, and more advanced unmanned aircraft.
Russia's failure to get Ukraine to waste its advanced air defense missiles comes alongside Russia's inability to destroy Ukrainian air defence systems, the UK MOD added.
The MOD said last month that Russia was prioritizing trying to knock out Ukraine's advanced air defense systems after Ukraine shot down Kinzhal missiles that Russia had previously bragged were unstoppable.
Ukraine has a variety of air defense systems, including the advanced Patriot missile systems it received from the US and European allies.
Last month, Russia claimed to have destroyed one Patriot missile system, but the US said it was still functional and had been quickly repaired.
The UK MOD also said that Russia was likely trying to "locate and strike Ukrainian forces well behind the front line," but without much success.
"Russia remains very ineffective at hitting such dynamic targets at range because of its poor targeting processes," it said.
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