NEAR BAKHMUT, Ukraine, May 18 (Reuters) - The Ukrainian military and Russia's Wagner private army both reported further Russian retreats around the city of Bakhmut on Thursday, as Kyiv pressed on with its biggest advance for six months ahead of a planned counteroffensive.

Ukrainian troops near the front line said Russia was bombarding access roads to slow the Ukrainian assault, which has shifted momentum after months of slow Russian gains in Europe's deadliest ground combat since World War Two.

"Now, for the most part, as we have started to advance, they are shelling all the routes to front positions, so our armoured vehicles can't deliver more infantry, ammunition and other things," said Petro Podaru, commander of a Ukrainian artillery unit.

Ukraine's military said troops had advanced in places by more than a mile. Its forces had been on the defensive for half a year, weathering a huge offensive by Moscow that saw only slow gains.

"Despite the fact that our units do not have an advantage in equipment ... and personnel, they have continued to advance on the flanks, and covered a distance of 150 to 1,700 metres (1.1 miles)," military spokesperson Serhiy Cherevatyi said in televised comments.

Ukraine's gains have been accompanied by a deepening public split within Russia's forces between Wagner, which has led the Bakhmut campaign, and the regular Russian military.

The blasted ruins of Bakhmut, described by both sides as a "meat grinder", would be Moscow's only prize for its huge winter offensive that failed elsewhere along the front.

Kyiv says it has launched local advances around Bakhmut as a prelude to an upcoming big counteroffensive that it hopes will turn the tide against Russia's 15-month-old invasion.

Wagner boss Yevgeny Prigozhin says his forces inside Bakhmut itself are still advancing, on the cusp of pushing Ukrainian troops out of their last foothold in the built-up area on the city's western outskirts.

But he accuses commanders of Russia's regular forces of abandoning ground north and south of the city, raising the risk of troops inside being encircled.

"Unfortunately, units of the Russian Defence Ministry have withdrawn up to 570 metres (1,880 feet) to the north of Bakhmut, exposing our flanks," Prigozhin said in his latest voice message on Thursday.

"I am appealing to the top leadership of the Ministry of Defence - publicly - because my letters are not being read," Prigozhin said, addressing Defence Minister Sergei Shoigu and Chief of the General Staff Valery Gerasimov.

"Please do not give up the flanks."

The Russian defence ministry has acknowledged some withdrawals from positions near Bakhmut over the past week but denies Prigozhin's assertions that flanks are crumbling, or that it has withheld ammunition from Wagner.