Russia is trying to destroy Ukraine’s electricity network before the winter but is struggling with a shortage of high-precision, long-range missiles, Western officials said Tuesday.

Ukraine says Russia has destroyed almost a third of its power stations through missile attacks launched in the last week, causing power cuts across the country including parts of Kyiv and the southern city of Mykolaiv.

Western officials said the Kremlin was “pursuing a deliberate strategy of attempting to destroy Ukraine’s electricity network” but was increasingly relying on Iranian Shahed (martyr) drones because its stockpiles of cruise missiles were running low.

“Our assessment is that the ability for the Russians to continue with these saturating barrages of precision weapons is getting to the point now where this will be unsustainable,” one official said.

Ukrainian forces are “effectively neutralizing many” Iranian drones before they hit key strategic targets in the country, the official added.

“They do fly quite slowly though so they are much more susceptible to being shot down by small arms and other systems. The tactics have been: Using them en masse in the hope some get through,” the official said.

In an update earlier this week, the U.K.’s defense ministry questioned Russia’s capability of launching long-range missile attacks, arguing the Russian defense industry “is probably incapable of producing advanced munitions at the rate they are being expended.”

Earlier Tuesday, U.K. Foreign Secretary James Cleverly criticized Russian President Vladimir Putin for launching “cowardly drone attacks.”

“These are desperate acts of a man losing a war on the battlefield,” he tweeted. “It’s why we sent air defence missiles. He won’t break the Ukrainians or our resolve to stand with them.”

The Western official also said the Russian forces are being led by a very small number of inexperienced officers, quickly promoted despite their lack of battlefield experience.