MOSCOW (AP) — A St. Petersburg court on Monday ordered the detention of two Russian men 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.

Videos released by the FSB showed its agents rounding up the two suspects and searching their homes.

Putin's spokesman Dmitry Peskov said Monday that the information provided by the U.S. was a "vivid proof of the efficiency" of counter-terrorism cooperation between the two countries.

In December 2017, Putin similarly thanked Trump for a CIA tip that helped thwart a series of bombings in St. Petersburg. The Kremlin said then that the CIA information led the FSB to nab a group of suspects that planned to bomb St. Petersburg's Kazan Cathedral and other crowded sites.