A city bus was hit by an artillery shell in Donetsk, eastern Ukraine, killing up to 13 people.

The shell struck the vehicle, instantly killing passengers and blowing out the windows of a nearby building.

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People look at a city bus that was hit by an artillery shell in DonetskAlexander Ermochenko/Reuters
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The interior of the trolleybus in Donetsk that was hit by an artillery shellAlexander Ermochenko/Reuters
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A man looks at a burnt car near a trolleybus stop in DonetskAlexander Ermochenko/Reuters

It was unclear on Thursday (22 January) who was responsible for the attack. Pro-Russian rebels blamed the Ukrainian army.

Ukrainian Prime Minister Arseny Yatseniuk blamed the rebels. At a ceremony to mark Unity Day in Kiev, he said: "Russian terrorists have today committed another horrible act against humanity. And responsibility for that lays on the Russian Federation."

Fighting between Ukrainian government troops and separatist forces surged after the New Year following a month of relative tranquility after a truce was declared in early December.

Last week, 13 civilians were killed as a commuter bus from Donetsk was hit by a shell in the near town of Volnovakha.

Kiev government forces have withdrawn from the new terminal at the main airport in Donetsk, a symbolic target where heavy fighting has been taking place for months. Six Ukrainian servicemen had been killed in the past 24 hours in fighting there, Kiev officials said.

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A masked man salutes on Independence Square in Kiev during the funeral ceremony for Tomaz Sukhiashvili, 35, a Georgian member of the Donbass self-defence battalion who was killed in the fighting in eastern UkraineGleb Garanich/Reuters
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Bogdanna Nikonenko holds a portrait of her father Sergiy on Independence Square in Kiev as she takes part in the funeral ceremony for the serviceman from the Aydar battalion who was killed in the Luhansk region. Pro-Russian separatists renewed attacks on Ukrainian forces at an airport complex in the east on Monday after Kiev launched a mass operation to reclaim lost ground thereGleb Garanich/Reuters
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A Ukrainian serviceman is seen during fighting with pro-Russian separatists in Pesky village, near DonetskReuters
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A Ukrainian serviceman fires a weapon during fighting with pro-Russian separatists in Pesky village, near DonetskReuters
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Residents clear up debris at a house damaged by recent shelling in Donetsk, eastern UkraineAlexander Ermochenko/Reuters
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Emergency workers inspect a damaged railway bridge near Kuznetsovka village in the Zaporizhzhya region, cutting a railway link between the city of Mariupol and Ukraine's westReuters
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New volunteers for the Ukrainian Interior Ministry's Azov battalion line up in Kiev before they head to the front lines in eastern UkraineGleb Garanich/Reuters
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New volunteers for the Ukrainian Interior Ministry's Azov battalion say goodbye to their loved ones in KievGleb Garanich/Reuters
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A new Azov battalion recruit embraces his girlfriend before departing for eastern UkraineGleb Garanich/Reuters

A law is being prepared to increase the size of the Ukrainian armed forces by 68,000 personnel to a total of 250,000, Yatseniuk said on Wednesday. Of this figure, 204,000 would be soldiers.

More than 4,700 people have died because of the conflict since April last year.