U.S.-supplied long-range artillery has helped stabilize the front line with Russia in the east of the country, the commander-in-chief of Ukraine’s armed forces said, as his troops geared up for a counteroffensive to retake territory in the south.

After weeks of grueling combat in eastern Ukraine that culminated in Russian forces claiming control over the Luhansk region, the arrival of U.S. mobile rocket launchers, known as Himars, last month has strengthened Kyiv’s hand.

“It is difficult, tense, but completely under control,” said Commander-in-Chief of the Armed Forces of Ukraine Valeriy Zaluzhny, who said he discussed the situation on the battlefield with Mark Milley, chairman of the U.S. Joint Chiefs of Staff. In a post on his Telegram channel Tuesday, Mr. Zaluzhny cited Himars as an “important factor contributing to our retention of defensive lines and positions.”