A top Chechen official said Wednesday that he was alarmed by President Boris Yeltsin's extension of an emergency security zone to Chechnya's borders and renewed charges that Moscow was planning a nuclear explosion in the breakaway republic."It naturally arouses alarm that Russian troops will be directly adjoining the borders of Chechnya," said the republic's Information Minister Movladi Udugov, referring to Yeltsin's Monday decree widening a security zone to halt ethnic violence in Chechnya's North Caucasus neighbor, Ingushetia. "It increases tension and increases danger."The political temperature in Chechnya, always high since the region declared its independence from Moscow in 1991, has reached the boiling point following an assassination attempt against Chechen President Dzhokhar Dudayev last week that he narrowly escaped. The attempt claimed the lives of the republic's two top police officials. The attack led Dudayev to impose a curfew throughout Chechnya on Wednesday. The Chechen leader, who has frequently made alarmist statements since 1991, charged earlier this week that the blast had been the work of special agents sent by Moscow and warned that the Kremlin was planning a nuclear explosion in Chechnya."Russia will organize a nuclear explosion on our territory and accuse the Chechens of being behind it," Udugov said by telephone from the Chechen capital Grozny on Wednesday, repeating Dudayev's allegation. The Russian Foreign Intelligence Service rejected the charges out of hand, Interfax reported Wednesday. Earlier this week, Russian officials denied involvement in the assassination attempt.The events of the last week have put an end to a brief thaw in the relations between Moscow and Chechnya, begun when Yeltsin's press secretary Vyacheslav Kostikov broached the possibility of a face-to-face meeting between Dudayev and the Russian president."It's only a game," Udugov said of Kostikov's overture. "On the one hand they say they want to talk, on the other they kill our people."Udugov, said that "in the next few days" Chechnya would hold military exercises. But he said that they had been planned before Yeltsin's decree and were not related to it.Yeltsin's decree Monday extends an emergency zone created in April 1991 in an area disputed by the Moslem Ingush and Christian Ossetians. The zone now takes in the Malgobek region and most of the Sunzha region in Ingushetia, which border Chechnya. The state of emergency, which includes a curfew, censorship and ban on public meetings is enforced by Russian soldiers and Interior Ministry troops.The zone was last widened in November 1992, when fighting broke out between the Ingush and Ossetians. At that point, Russian troops in Ingushetia amassed along the border with Chechnya.Chechnya and Ingushetia once formed a single republic, but split in 1992 when Ingushetia resisted Chechnya's self-declared independence and chose to stay in the Russian Federation.