By Joseph Campbell and Zohra Bensemra

SUMY, Ukraine (Reuters) - Russia's forces may have pulled back in some parts of Ukraine more than a week ago, but the territorial defense force in the northern Sumy region is training and bracing for further attacks.

Shortly after Russian forces invaded the country on February 24 in what President Vladimir Putin called "a special military operation", they crossed the border into Sumy, fighting in the streets of towns as they moved towards the capital Kyiv.

Dmytro Zhivitskyi, head of Sumy's regional and military administration, said at that time there were almost no regular army troops, instead locals took up whatever arms they could find, such as Molotov cocktails.

"According to the Russians, they had plans [to take over Sumy] in three to five days," Zhivitskyi told Reuters in Sumy on Thursday.

"Apparently they knew that in the territory of Sumy region at that time there were almost no regular [army] troops, and there was only territorial defense."

Ukrainian forces retook control of the northeastern region on April 8, Zhivitskyi said.

"I think the probability of a [new] attack is high. They are determined and we understand that the number of people in Russia is about 150 million," he said. "Until the tanks and people run out, they will keep sending people here."

The Russian invasion has left a trail of death and destruction that has drawn worldwide condemnation and triggered concern about Putin's broader ambitions.

Russia has dismissed allegations of its troops committing war crimes in Ukraine as fake news.

Western countries say Russia is now reinforcing and resupplying its troops for an intensified offensive in eastern Ukraine. It is not clear whether the Kremlin plans to attack the Sumy region again.

But the local territorial defense force is getting prepared, with members saying the volunteer militia now stands at more than 1,000 people. Local authorities would not say how many regular troops were in Sumy.

Ihor Hannenko, a former youth worker-turned territorial defense member, joined up on day one of the war. On Friday, Hannenko and several members of his brigade carried out communication drills, holding training simulations for a raid at an abandoned building on the city's outskirts.

"There were a lot of situations when we would go on a mission, and we were called and told that there are no Russian columns there because someone had already destroyed them," said the 28-year-old. "We didn't even know who did it."

(Writing by Elizabeth Piper in Kyiv; editing by Hugh Lawson)