Kharkiv (Ukraine) (AFP) - The city of Kharkiv in Ukraine's industrial east has stayed out of rebel hands but a mysterious spate of bombings has triggered fears that Russia is trying to destabilise this key defence hub.

Barely a week goes by without another bombing reported against Ukrainian military targets in the city of 1.4 million that once served as Ukraine's capital.

Most have been low-level attacks with few victims, but the targets are symbolic -- a National Guard base on December 2, an anti-aircraft unit and military hospital in late November.

Investigators say they often find traces of plastic explosive -- common during Soviet times and often sourced from Russia.

Plastic explosive was used to bomb a bar collecting money for Ukrainian soldiers in the city centre on November 10, leaving 11 wounded and the building destroyed.

The city is known as a hub of the Ukrainian defence industry -- with several aerospace manufacturers and the factory that builds the Malyshev tank.

In one of the more dramatic attacks, insurgents used flamethrowers against a building at the Malyshev factory and a military base in August.

Police arrested six Kharkiv locals with flamethrowers, which they said were sourced from Russia, along with anti-tank mines and Kalashnikov rifles.

Ukrainian security services (SBU) also claimed to have foiled a plot to kill Dutch Foreign Minister Bert Koenders when he was visiting the Malyshev factory in November.

The factory was being used to store the remains of victims from the MH17 commercial airliner that was shot down over the conflict zone in July with 298 people on board.

The SBU released a video confession of a woman saying she had been recruited by Russia's military intelligence to lay explosives along the route of the Dutch delegation.

In the heightened tensions as a result of the war in the east and the vicious diplomatic tussle between Ukraine and Russia, it has become difficult to verify the allegations.

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Russia denies interfering in the rebellions of eastern Ukraine, but a wealth of evidence from NATO, Kiev and Western governments suggests otherwise.

Kharkiv's defence industries make it a prized target for both Ukrainian rebels and the Kremlin, analysts say.

The attacks "are aimed at sowing panic and destabilising this strategic region", said Anatoliy Oktysyuk of the Centre for International Political Studies in Kiev.

"In Kharkiv, Ukraine is facing a new type of attack... which show they are well-planned and carried out by professionals."

Unlike provinces further east, Kharkiv did not descend into full-blown rebellion against the new pro-Western government, even though pro-Russian protesters did briefly seize control of government buildings in the early days of the insurgency in April.

But Oktysyuk warned that the region remains vulnerable if Russia manages to infiltrate local power structures or destabilises the city with a steady stream of attacks.