KYIV, June 12 - Ukrainian troops are probing Russian defenses as spring gives way to a second summer of fighting, and Kyiv’s forces are facing an enemy that has made mistakes and suffered setbacks in the 15-month-old war.

But analysts say Moscow also has learned from those blunders and improved its weapons and skills, The Associated Press reported.

Russia has built heavily fortified defenses along the 1,000-kilometer (600-mile) front line, honed its electronic weapons to reduce Ukraine’s edge in combat drones, and turned heavy bombs from its massive Cold-War-era arsenal into precision-guided gliding munitions capable of striking targets without putting its warplanes at risk.

The changing Russian tactics along with increased troop numbers and improved weaponry could make it challenging for Ukraine to score any kind of quick decisive victory, threatening to turn it into a long battle of attrition.

U.S. Joint Chiefs chairman Gen. Mark Milley said in an interview with The Associated Press on Tuesday that while Ukraine’s military is well-prepared, as time goes on, “this will be a back-and-forth fight for a considerable length of time.”

President Volodymyr Zelenskyy said Saturday that counteroffensive and defensive actions are underway against Russian forces, asserting that his commanders are in a “positive” mindset about its success. Ukrainian authorities have stopped short of announcing the start of a full-blown counteroffensive.

Sir Richard Barrons, a retired general who led the U.K. Joint Forces Command, said the Russian military has built “textbook” defensive lines and adjusted its tactics following its hasty retreat from wide swaths of the Kharkiv and Kherson regions last fall under the brunt of a swift Ukrainian campaign.

He pointed at the improved Russian ability to both counter and use drones and also noted that Moscow has learned to keep key assets like command headquarters and ammunition dumps out of artillery range.

“And they have sharpened up how they can fire at Ukrainian artillery and tanks when they spot them,” he told AP. “So if you add all that together, everybody knows this will be a harder fight than for Kherson or Kharkiv in the autumn of last year.

Russian troops continue to suffer from low morale, there are shortages of ammunition, and coordination between units has remained poor. Vicious infighting has erupted between the military brass and the Wagner private military contractor, which has fielded tens of thousands of mercenaries to the battlefield to spearhead the battle for Bakhmut.

A major factor still limiting Russia’s capability has been its decision to keep its air force from forging deep into Ukraine after it suffered heavy losses in the war’s initial stages. Its attempts to knock out Ukraine’s air defenses have failed. Thanks to supplies of Western weaponry, Ukraine now poses an even more formidable challenge to Russian aircraft.

Barrons emphasized it’s essential for military leaders in Kyiv to continue keeping its adversary’s warplanes at bay so that “the counteroffensive isn’t the moment the Russian air force suddenly finds its capability and courage and romps ... all over Ukraine.” (ap/ez)