DONETSK, Ukraine - Deadly clashes between Ukrainian forces and pro-Russian rebels threatened civilians in central Donetsk on Monday, with shelling hitting a hospital and warnings of insurgents launching attacks from residential areas.

Ukraine also made fresh accusations of Moscow backing the separatists, alleging that hundreds of Russian troops crossed the border on Monday into rebel-held territory to join the fight.

After a pause in the fighting early Monday as Orthodox Christians celebrated Epiphany, clashes reignited in the afternoon, with regular explosions heard coming from the direction of the Donetsk airport northwest of the city.

The airport has been the flashpoint of an upsurge in violence in recent days that has left a September truce in tatters, with heavy combat shaking the area at the weekend after Ukraine launched a major counter-offensive to push out the rebels.

The hospital hit on Monday is however located in the centre of Donetsk, and rebel officials who control the area reported six wounded, including one doctor and five patients. The building’s front was damaged and windows were blown out.

A university across from the hospital was also hit and there was speculation in the neighbourhood over whether the intended target was the nearby separatist security ministry.

“I walked out of a store and took two steps, then I heard ‘boom! boom! boom!’” said 35-year-old Denis Rybkine. “By reflex, I lay down on the ground, and I saw red bricks (from the hospital) fly and windows explode.”

Larissa Polyakova said students at the university barely missed being hit.

“Shells fell on the window,” she said. “Miraculously, the students had left the room 15 minutes before. Can you imagine what would have happened if they were still inside?”

Elsewhere, in Debaltseve some 60 kilometres (around 40 miles) north of Donetsk, artillery fire killed three people on Monday and wounded 12, according to Ukrainian officials.

Concerns mounted over the fate of civilians, with rebels said to be launching attacks from residential areas and Ukrainian forces accused of returning fire.

“The use of heavily populated areas for launching attacks by pro-Russian separatists in Donetsk and Horlivka and the return of fire into these areas by pro-Kiev forces is putting civilian lives in great danger,” Denis Krivosheev, deputy Europe and Central Asia programme director for Amnesty International, said in a statement.

“Pro-Russian separatist forces must stop using densely populated areas for launching military operations and Kiev-controlled forces must not launch indiscriminate attacks which put civilian lives at risk.”

Ukraine’s military reported at least three soldiers killed over the last 24 hours and another 66 wounded, but claimed to be in control of the airport.

Rebels have disputed Kiev’s account and there was no independent confirmation, with journalists unable to approach the site.

The blackened and wrecked airport has been hit by repeated on-off clashes, and control has often been divided between a new and old terminal and other installations.

With rebels at one point claiming to control the airport, Ukraine launched a massive counter-offensive with tanks and other heavy artillery on Saturday that continued into Sunday.

Intense clashes followed, with repeated heavy explosions ringing out from the area surrounding the airport, while some were also heard closer to the city centre.

Kiev and Moscow traded blame for the breakdown of the ceasefire as the airport battle raged, while UN Secretary General Ban Ki-moon voiced alarm and called for an end to the violence.

Ukraine and Western nations say Russia has supported the rebels with troops and weapons, charges Moscow strongly denies despite witness claims to the contrary.

Kiev issued further accusations on Monday, saying some 700 Russian troops had crossed the border into rebel-held territory earlier in the day.

“This morning, two groups of armed forces from the Russian Federation crossed the border,” military spokesman Andriy Lysenko told AFP.

Russia is under heavy sanctions from the West over its alleged actions in Ukraine, and the UN Security Council is due to discuss the worsening crisis at a meeting on Wednesday.

The conflict has killed more than 4,800 people since April last year and has become Europe’s worst humanitarian crisis since the Balkan wars of the 1990s.