Story highlights Raid is in the eastern city of Luhansk, where the group was allegedly plotting attack

15 were detained, accused of betraying the government, weapons violations

Authorities say they seized 300 guns, knives and a grenade launcher

U.S. Defense Secretary Chuck Hagel says the "world will respond" to Russia's actions

Ukrainian authorities have dismantled an armed underground group they claim was plotting against the Kiev-based government and planned to launch an attack in a few days, that nation's top security agency announced Saturday.

Fifteen people were detained following a large-scale operation in the eastern city of Luhansk, the Security Service said in a statement. The raid also netted about 300 guns, a grenade launcher as well as numerous grenades, Molotov cocktails and a significant amount of knives, according to the agency.

The Security Service alleges the group was plotting to carry out an attack April 10 in Luhansk. Those detained face charges that include betraying the government and weapons violations.

Luhansk is on the border with Russia in eastern Ukraine, a region that some say could follow the lead of Crimea and soon fall under Moscow's control. The city has a substantial Russian minority.

A classified U.S. intelligence assessment last month identified Luhansk, along with Kharkiv and Dontesk, as possible targets for Russian forces seeking to establish a land bridge to Crimea.

NATO says tens of thousands of Russian troops have massed near Ukraine's border, amid fears they may seek to invade. Russia says its forces are just on military exercises.

Those fears come on the heels of the ouster of Ukraine's pro-Moscow president was ousted and the emergence of a new Western-leaning government in Kiev. That was followed by Crimea leaving Ukraine and being folded into Russia following a quickly organized referendum -- a move that derided by authorities in Kiev and officials in the West.

Moscow's recent moves have caused the worst crisis in East-West relations since the end of the Cold War.

The United States and European Union have responded with asset freezes and travel bans against individuals and warned of further economic and diplomatic sanctions if Russia does not curb its activities.

Speaking to reporters Saturday ahead of his arrival in Japan, U.S. Defense Secretary Chuck Hagel said that the "world will respond" to Russia's annexation of Ukraine.

"I think any time you have a nation, Russia in this case, try to impose its will to define international boundaries and violate territorial integrity and sovereignty of a nation by force, all of the world takes note of that," he said.

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"All of the world should take note of that, and the world will respond to that."