A St. Petersburg court on Monday ordered the detention of twoRussianmen who were arrested on a tip provided by the U.S. and are suspected of plotting unspecified terrorist attacks in the city during the New Year holidays.

The Dzerzhinsky District Court ruled that the suspects identified as Nikita Semyonov and Georgy Chernyshev should remain in custody pending their trial.

The Federal Security Service (FSB), the main KGB successor agency, said in a statement that the suspects detained on Friday confessed to plotting the attacks. It added that it also seized materials proving their guilt.

The FSB didn’t elaborate on their alleged motives or targets, but Russia’s state television reported that the suspects had recorded a video swearing their allegiance to the Islamic State group.

The FSB said it was acting on a tip provided by its “American partners.”

On Sunday, Russian President Vladimir Putin called U.S. President Donald Trump to thank him “for information transmitted through the special services that helped prevent terrorist attacks in Russia,” according to the Kremlin.