(CNN) -- 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.
This announcement came out the same day that U.S. Defense Secretary Chuck Hagel said that the "world will respond" to Russia's annexation of Ukraine's Crimea region, which has heightened tensions not only in Ukraine but around the world.
Hagel, speaking to reporters ahead of his arrival in Japan, was asked what reassurance the United States could give to its allies in light of Russia's recent actions.
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"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.
"All of the world should take note of that, and the world will respond to that."
Nations need to stay alert to this threat, and allies will look to each other and want to be reassured, he added.
Moscow's annexation of Crimea last month, condemned as illegal by the West, has 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.
Tensions remain high thanks to the tens of thousands of troops that NATO says Russia has near Ukraine's eastern border, amid fears they may seek to invade. Russia says its forces are just on military exercises.
Japan is part of the G7, a group of the world's leading industrialized nations that backed steps to isolate Russia by excluding it from what was the G8. The G7 will meet in June in Brussels, Belgium, instead of attending a planned G8 summit in the Russian city of Sochi.
But Japan also has its own concerns closer to home.
Tokyo and Beijing's bitter dispute over a set of small, uninhabited islands in the East China Sea has led to frequent tense encounters between the two sides' ships and planes in recent times.
Asked about tensions between Japan and China over territorial issues, Hagel said it was up to individual nations to resolve their disputes, but the United States would always stand by its treaty obligations with Japan.
Hagel, in Japan for two days of talks, called the Japan-U.S. relationship one of the strongest partnerships his nation has and "a very critical anchor to peace and stability and security in this part of the world."
Military cooperation has strengthened over the past couple of years, he said, and the United States is boosting its commitment to Japan.
China, too, "is an important relationship, an emerging relationship with the United States," Hagel said.
The United States has thousands of troops stationed in Japan, and U.S. officials have reiterated that a mutual defense treaty between the two countries applies to the disputed islands, known as Senkaku by Japan and Diaoyu by China.
Hagel's visit to Japan comes on the heels of three days of talks with defense ministers from the Association of Southeast Asian Nations, at the U.S.-ASEAN Defense Forum in Hawaii. He will also meet with leaders in China.
CNN's Greg Botelho and Elena Sandyrev contributed to this report.