Ukraine's state security service has detained 15 people suspected of planning to overthrow the authorities in a mostly Russian-speaking eastern region. They confiscated hundreds of rifles, grenades and petrol bombs.

The service said those arrested were planning to stir up unrest in the region of Luhansk which, like most of Ukraine's eastern regions, has been tense since the ouster of pro-Russia former president Viktor Yanukovich in February.

"The group of attackers planned to carry out an armed seizure of power on April 10 in the Luhansk region through the intimidation of the peaceful population and the use of weapons and explosives," the service, which has intelligence and policing functions, said in a statement.

Eastern Ukraine's population is largely made up of Russian-speakers who are culturally close to Russia. Their regions have become touchpoints for confrontation between the two countries since Yanukovich fled after months of protests against him.

Tensions have risen since Russia's annexation of Ukraine's Crimea territory in March, a move which has led to the biggest confrontation between Russia and the west since the end of the Cold War.

Pro-Russian demonstrators held rallies in eastern Ukrainian cities in recent weeks, not far from the border with Russia where Moscow has gathered troops and boosted their numbers to tens of thousands.

Kiev has accused Russia of sending Russian citizens to Ukraine to foment unrest in eastern and southern regions and has instituted stricter controls on its border with Russia.

Likewise, Russia's Federal Security Service said earlier this week it had detained 25 Ukrainians suspected of preparing attacks in the southern and central part of the country.

Ukraine's security force said it was carrying out more arrests in Luhansk and seven other regions and that it had confiscated 300 automatic rifles, an anti-tank gun and large numbers of grenades, petrol bombs and knives.