Ukraine's troops are probing Russian defences as spring gives way to a second summer of battle, and Kyiv's forces are up against an opponent who has made mistakes and suffered defeats during the 15-month conflict. However, observers believe Moscow has learnt from its mistakes and upgraded its weaponry and capabilities.

Russia has fortified its defences along the 1,000-kilometer (600-mile) front line, honed its electronic weapons to counter Ukraine's advantage in combat drones, and converted heavy bombs from its massive Cold War-era arsenal into precision-guided gliding munitions capable of striking targets without endangering its warplanes.

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.”

Most attention last week focused on catastrophic flooding in southern Ukraine caused by the destruction of the Kakhovka dam that both sides blame on each other.

At the same time, however, Ukrainian troops have unleashed a series of attacks in several parts of the front that so far have made only marginal gains against multilayered Russian defenses.

Ukrainian 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.

A day earlier, Russian President Vladimir Putin claimed it had begun, but Ukraine had made little progress and had suffered "significant" losses.

Sir Richard Barrons, the retired general who led the United Kingdom's Joint Forces Command, said the Russian military has built "textbook" defensive lines and adjusted its tactics in response to its hasty retreat from large swaths of the Kharkiv and Kherson regions last fall under the brunt of a swift Ukrainian campaign.

He emphasised Russia's enhanced capacity to counter and employ drones, as well as how Moscow has learnt to keep important assets like command headquarters and munitions 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.

“People are still using those two successes, and they were successes, as benchmarks, which I think is unfair, unreasonable in the circumstances,” he said.

Russia has fielded more troops to protect the long front line, even though many of them could be poorly trained, he said.

During a botched effort to conquer Kyiv at the outset of the war, Russian military convoys extended for miles, becoming easy prey for Ukrainian artillery and drones.

In the autumn, Ukrainian missiles sunk the Russian cruiser Moskva, the flagship of the Black Sea Fleet, inflicting a huge blow to Moscow's prestige; Kyiv's rockets pounded Russian ammo dumps and command headquarters; and Kremlin forces withdrew quickly from vast territories in the east and south.

Despite these failures, Russia dug in to defend large swaths of Ukrainian territory taken early in the invasion. After the war's longest and deadliest combat, it took control of the eastern city of Bakhmut last month.

Fundamental Russian weaknesses remain.

Russian forces continue to suffer from low morale, ammunition shortages, and poor coordination between units. Violent infighting has developed between military brass and the Wagner private military contractor, which has sent tens of thousands of mercenaries to the battlefield to lead the struggle for Bakhmut.

One important issue that continues to limit Russia's potential is its determination to restrain its air force from advancing far into Ukraine after suffering catastrophic casualties in the early days of the war. Its efforts to disable Ukraine's air defences have failed. Ukraine now offers an even more dangerous challenge to Russian aircraft, thanks to supply of Western armament.

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.”

Ukrainian military analyst Oleh Zhdanov notes that Moscow has maintained a numerical advantage in troops and weapons, despite any weaknesses.

While Russia has increasingly tapped its Cold-War arsenals, deploying tanks dating to the 1950s to replenish its massive, early losses, such old weapons can still perform well, Zhdanov said.

“It doesn’t matter what tanks they have; they have thousands of them,” Zhdanov told AP, noting Russia put many of them to use as stationary weapons in their defensive lines, including in the Zaporizhzhia region where they proved effective.

He acknowledged Russian success in hitting Ukrainian military depots. relying on Moscow’s agents and collaborators, but said such losses were “tolerable.” He also said the Russians increasingly use drones and improved electronic warfare to jam those from Ukraine.

According to Zhdanov, Russia has transitioned away from the battalion-sized tactical groupings that were used early in the war and towards smaller forces.

While the Russian air force has operated in modest numbers, he claims that it has modernised its supply of bombs to transform them into gliding weapons that have proven effective. The 500-kilogram (1,100-pound) GPS-equipped bombs may do huge damage.

"The Soviet Union produced those bombs in uncountable numbers," Zhdanov explained, adding that the Russians drop up to 50 each day for a "major psychological effect."

In April, a similar bomb was unintentionally unleashed over the Russian city of Belgorod, near the Ukrainian border, causing a massive crater and injuring one person mildly.

Russian military bloggers hailed the punch of the gliding bombs and their ability to hit targets up to 70 kilometers (over 43 miles) away. One former military pilot said in his blog that work is under way to convert 1,500-kilogram (3,300-pound) bombs into gliding munitions.

These conversions allow the Russian air force to ramp up strikes on Ukrainian forces without risking its warplanes.

The Royal United Service Institute, a London-based think-tank that focuses on defense and security issues, listed these gliding bombs along with other improvements in Russian weapons and tactics.

“Although they only have limited accuracy, the size of these munitions poses a serious threat,” RUSI said in a recent report, adding Russia was working to improve their accuracy.

According to the study, Russian engineers have demonstrated their ability to construct field fortifications and sophisticated obstacles along the front line, such as concrete-reinforced trenches and command bunkers, wire-entanglements, ditches, anti-tank hedgehogs or 'dragon's teeth' and complicated minefields.

The extensive deployment of sophisticated explosives against tanks and troops poses "a major tactical challenge to Ukrainian offensive operations," according to the RUSI writers.

Other Russian advancements mentioned in the report include improved thermal camouflage for tanks, more agile deployment of artillery into multiple positions, including integration with drones to avoid losses, and attacking Ukrainian artillery with loitering munitions — drones that hover until they acquire a target.

Such retaliatory Russian fire poses "the greatest challenge to Ukrainian offensive operations," according to the RUSI assessment.

Improved Russian electronic warfare systems have destroyed about 10,000 Ukrainian drones a month, while they also have been able to intercept and decrypt Ukrainian tactical communications in real time, it added.

They also have learned to intercept GPS-guided rockets fired by Western-supplied launchers like the U.S.-made HIMARS, which embarrassed the Russians and inflicted major damage, the report said.

Russia’s military “is able to improve and evolve its employment of key systems,” RUSI said, but noted it could struggle to respond to similar quick adjustments by Kyiv that could make Moscow’s units ”likely to rapidly lose their coordination.”