(KYIV, Ukraine) — Ukraine on Thursday began its long-awaited counteroffensive against Russia, officials told ABC News.

Well-trained Ukrainian troops had been gathering at strategic locations near the front lines in recent days, Western officials said last week.

Two Ukrainian officials, including a source close to President Volodymry Zelenskyy confirmed to ABC News that an active phase of the Ukrainian counteroffensive is underway.

Arguably, Ukraine’s counteroffensive was getting going a few days ago, and the Institute for the Study of War said as much on Monday, saying on Twitter that “Russian and Ukrainian officials are signaling the start of the Ukrainian counteroffensive.”

However there have been significant developments over the past few hours. Multiple reports said a major battle has begun in southeastern Ukraine, south of the major Ukrainian city of Zaporizhzhia.

Well-placed unofficial pro-Ukrainian sources have said the southeastern front is becoming more active and there are unconfirmed images and reports that Ukraine’s new modern German-made Leopard 2 tanks are involved in the offensive.

On Thursday morning, the head of Russia’s Wagner mercenary group, Yevgeny Prigozhin, said “the events that are happening now on the front line signal the start of the offensive and Ukraine will intensify its efforts.”

Copyright © 2023, ABC Audio. All rights reserved.