DONETSK, Ukraine — Ukrainian officials said their forces advanced to the outskirts of a key town north of Donetsk on Saturday as they try to retake the stronghold held for months by pro-Russia rebels.

The move comes as Ukrainian forces appear to have gained some momentum recently by retaking control of territory from the rebels. But Russia also appears to becoming more involved in the fighting, with the U.S. and Ukraine accusing Moscow of moving heavily artillery across the border to the rebels.

Ukrainian national security spokesman Andriy Lysenko said Ukrainian forces were outside Horlikva, just north of the regional center of Donetsk.

Once they can take Horlivka, “the direct route is open for the forces of the anti-terrorist operation to the capital of the Donbass region – the city of Donetsk,” Lysenko said. “The approaches to Donetsk are being blocked so that the terrorists do not get the chance to receive ammunition, reinforcements or equipment.”

Donetsk is a major center of the separatist uprising that has battled Ukrainian government forces for five months.

The Associated Press found the highway north of Donetsk blocked by rebels and heard the sound of artillery to the north. About 35 miles to the east, the site where Malaysia Airlines Flight 17 was shot down on July 17 was still eerily empty.

The disaster sparked hopes in the West that Russia would scale back its involvement in the uprising in Ukraine’s east, but the opposite seems to be the case. Russia launched artillery attacks from its soil into Ukraine on Friday, while the U.S. said it has seen rocket systems moving closer to the Ukraine border.