LYMAN, Ukraine (AP) — Concealed under pine branches in the forests of northeast Ukraine, the muzzle of a Soviet-era howitzer rises, aiming for a group of approaching Russian infantrymen many kilometers away.

A Ukrainian soldier signals to fire, then swiftly runs for cover. The thunderous crash of the unleashed projectile sends a pall of black smoke billowing above jabs of yellow flames. A pile of spent shells in the nearby foliage grows by the day.

Here, along a small section of the 1,200-kilometer (745-mile) front line, Moscow’s army is staging a ferocious push designed to pin down Ukrainian forces, distract them from their grinding counteroffensive and minimize the number of troops Kyiv is able to send to more important battles in the south.

The Kremlin tactic threatens to further slow the pace of the counteroffensive that was launched almost three months ago. Kyiv’s effort to reclaim Russian-occupied territory has produced minimal gains and heavy losses, and time is running short for Ukrainian troops, who must try to make the most of the last few weeks of the summer fighting season.

The Ukrainian military now considers the battles in the northeast, specifically near the town of Kupiansk, in the northern Kharkiv region, and in the forests near Lyman, to be Russia’s main offensive.

At the same time, Ukraine’s main offensive operations are focused in the south, where they are inching toward the shores of the Sea of Azov in an apparent bid to cut the land corridor to the Crimean Peninsula, which Russia seized in 2014. Doing this would split in two the Russian-occupied land in southern Ukraine, undermining Moscow’s supply lines.

Deputy Defense Minister Hanna Maliar urged observers to measure Ukrainian progress not in kilometers or meters but “by the very fact that we are successful in moving forward in such conditions.”

While aiming to keep Ukrainian troops busy along the mostly static northeast front, Russia has also had time to reinforce its defenses in the south, including laying widespread mines, Ukrainian officials said. The deep fortifications have slowed Kyiv’s advances in that direction.

Meanwhile, the Ukrainians must deal with limitations in manpower, air power and artillery. And the looming fall rainy season adds even greater urgency to an already difficult battle. The muddy ground will hinder Kyiv’s infantry and heavy machinery.

In the south, Ukrainian forces have recently had more success breaking through Russian lines. Since the start of the counteroffensive, Ukraine has advanced 7 kilometers (4.3 miles) in the southern Zaporizhzhia region, overcoming dense Russian fortifications last week to retake the village of Robotyne — Ukraine’s first tactically significant victory in that part of the country.

It is a far cry from the sweeping territorial gains Western allies hoped for. But winning control of the village brings Ukrainian forces one step closer to the town of Tokmak, about 30 kilometers (19 miles) away, an important Russian-occupied rail hub that would be a major strategic gain. And if Ukrainians advance even 15 kilometers (9 miles) from Robotyne, it could bring them within shooting range of Russia’s east-west transport routes, potentially undermining Moscow’s combat capabilities, military observers say.

“We passed the first line of Russian defense, and we are approaching the second,” said a Ukrainian soldier with the call sign “Legion” who is positioned in Zaporizhzhia. He said the success was owed to NATO-supplied weapons, in particular U.S.-made Bradley combat vehicles, as well as Ukrainian-made drones capable of striking 60 kilometers (37 miles) behind Russian lines.

In some places, including the Robotyne area, the second defensive line “is quite strong,” military spokesman Oleksandr Shtupun said. “Without proper preparation, it is hard to breach it.”

Kyiv has never explicitly stated its goals for the counteroffensive, apart from saying it seeks to restore Ukraine’s 1991 territorial borders.

In the northeast, Russia intensified its operation in mid-July, amassing 100,000 troops. Dark patches of scorched trees mark where Russian artillery assailed Ukrainian positions in the lush woodlands near Lyman. Soldiers joke that the area locals dubbed the “silver forest” is a black forest now.

On the outskirts of Kupiansk, Ukrainian forces must move across mostly open fields near the Synkivka settlement, where Russian forces have focused their advance. Villages nearby have been destroyed, giving Ukrainian forces few options for cover.

“The enemy is constantly trying to advance,” said brigade artillery commander Viktor Yurchuk. “Fighting has been nonstop.”

Col. Gen. Oleksandr Syrskyi, commander of Ukraine’s ground forces, said Russian troops were regrouping in both the Kupiansk and Lyman areas and deploying newly formed brigades and divisions as well as weaponry. Maliar said professional airborne units have also been deployed from around Avdiivka, further south in the Donetsk region where the two armies are also locked in battles.

Apart from preventing Ukrainian forces from redeploying in the south, a Russian advance in the northeast would also create a protective buffer for the Kremlin’s supply lines. Moscow hopes to halt Ukrainian advances in Bakhmut, where Kyiv’s forces recently took control of commanding heights within closer range of Russia’s supply routes.

For Yurchuk, that means the intensity of the battles will not let up anytime soon. After 18 months of war, he is tired.

“Everyone is fed up with the war, believe me,” he said.

Privately, some of Ukraine’s allies have expressed concern that the counteroffensive may come up short. Soldiers respond that every kilometer of advance is a herculean feat against a well-fortified enemy.

US. national security adviser Jake Sullivan pushed back against the notion that Ukraine is in a stalemate, saying last week that Ukrainians “are operating according to their tactics and their timetable.”

The allies’ concerns have reached Ukrainian soldiers on the front line.

“It’s very hard for our soldiers to hear that our assault is going too slowly,” said a drone operator known by the call sign “Salam” with the elite Adam Group in the Bakhmut area. “We are here witnessing the situation on the front line, and we never expected something rapid.”

It’s a view many Ukrainian service members share.

Reflecting the frustrations, Ukrainian Foreign Minister Dmytro Kuleba said this week that criticizing the slow pace equals “spitting into the face of the Ukrainian soldier who sacrifices his life every day.”

“I would recommend (to) all critics to shut up, come to Ukraine and try to liberate one square centimeter by themselves,” he said while on a trip to Spain.

George Barros, an analyst with the U.S-based Institute for the Study of War think tank, challenged assessments that the counteroffensive is not going well.

“This campaign is going the best that it could have, given the way that it was supported,” he said.

Because the Russians appear to have limited flexibility in their reserves, even a small Ukrainian breakthrough that severs a strategic point in Russian lines might result in the Kremlin’s troops being “stretched very thin,” Barros said.

For now, there is no indication that Russia’s efforts in the northeast are having a significant impact on the Ukrainian offensive in the south.

“On time, let’s say we have around one more month” before the rains set in, Shtupun said. “I think we still have time for offensive actions, just like our enemy does too.”