KYIV/EASTERN UKRAINE, Ukraine - As buildings change hands in chaotic fighting in Ukraine's eastern stronghold of Pokrovsk, defending forces said this week that drones alone could not hold off Russian advances and more soldiers were needed to prevent the enemy infiltrating further.

The situation, they said, was difficult, but not yet fatal for Ukraine in a city Russian forces have been trying to capture for around 18 months.

Open-source maps show more than half of Pokrovsk, a key logistical hub in eastern Ukraine, as a grey area controlled by neither side, with Russian troops close to cutting off supply routes in and out.

In rare firsthand remarks from people engaged in the battle, three Ukrainian servicemen described urban clashes and fluid dynamics when they spoke to Reuters on Monday and Tuesday.

Hus, a drone squad commander from the 68th Jaeger brigade, said on a video call from his dugout on the outer edges of Pokrovsk that there was no conventional front line in the city.

"Here, one building is held by our side, the next occupied by the enemy, and in a few hours it swaps," he said on Monday, giving only his call sign in line with Ukrainian military practice.

Some servicemen Reuters spoke to said a shortage of troops was a key challenge as Ukraine seeks to hold the city - as it has been in other urban battles where Russia eventually prevailed.

SITUATION DETERIORATING BUT NOT CRITICAL

According to Ukraine's 7th Rapid Response Corps, Russia has at least 11,000 troops in the vicinity of Pokrovsk, with thousands more in surrounding areas. Ukraine is heavily outnumbered, but officials do not disclose details of Ukrainian troop levels.

Hus said that several days ago, his unit was moved from positions in Pokrovsk to the outskirts as the situation was "steadily deteriorating". Kyiv deployed special forces to the city last week in a sign of the precariousness of the situation.

Russia has been saying for over a week that Ukrainian troops in P...