The Kremlin on Friday demanded that Belarus quickly release 33 Russian private security contractors detained on terrorism charges, dismissing the accusations as bogus.

Belarusian officials said the employees of private Russian military contractor Wagner are facing a criminal probe on charges of plotting terror attacks in Belarus amid the country's presidential election campaign. The contractors were detained Wednesday.

The allegations represent an unprecedented escalation of tensions between Russia and Belarus, traditionally close allies, as Belarus' President Alexander Lukashenko seeks a sixth term in the Aug. 9 election. Independent observers and opposition supporters in Belarus see the detention of the Russians as part of the authoritarian Lukashenko's efforts to shore up sagging public support.

Maria Kolesnikova, a representative of Viktor Babariko, right, Svetlana Tikhanovskaya, candidate for the presidential elections, centre, and wife of non-registered candidate Valery Tsepkalo, Veronika Tsepkalo, left, gesture during meeting in support of Svetlana Tikhanovskaya in Minsk, Belarus, Thursday, July 30, 2020. Thousands of Tikhanovskaya's supporters gathered Thursday at her campaign rally in Minsk. The presidential election in Belarus is scheduled for August 9, 2020. (AP PhotoSergei Grits)

Maria Kolesnikova, a representative of Viktor Babariko, right, Svetlana Tikhanovskaya, candidate for the presidential elections, centre, and wife of non-registered candidate Valery Tsepkalo, Veronika Tsepkalo, left, gesture during meeting in support of Svetlana Tikhanovskaya in Minsk, Belarus, Thursday, July 30, 2020. Thousands of Tikhanovskaya's supporters gathered Thursday at her campaign rally in Minsk. The presidential election in Belarus is scheduled for August 9, 2020. (AP PhotoSergei Grits)

The Kremlin, which first reacted cautiously to Belarus' move, toughened its stance Friday. Kremlin spokesman Dmitry Peskov said Russia expects Belarus to quickly free the detainees.

“The unwarranted detention of the 33 Russian citizens doesn't fit into the framework of the allied relations,” Peskov said during a conference call with reporters. “We expect our Belarusian allies to clear up this incident in the nearest time and release our citizens.”

Peskov said members of a Russian security firm were en route to an unspecified country and missed a connecting flight to Istanbul at the Belarusian capital’s airport. “They have done nothing wrong and carried no illegal items,” he added.

Belarusians wearing face masks to protect against coronavirus, attend a meeting in support of Svetlana Tikhanovskaya, candidate for the presidential elections in Minsk, Belarus, Thursday, July 30, 2020. Opposition candidate Svetlana Tikhanovskaya said she would not cancel any campaign rallies because of the stricter security. Thousands of Tikhanovskaya's supporters gathered Thursday at her campaign rally in Minsk, and many participants dismissed the official claims of a Russian subversion plot as a sham. The presidential election in Belarus is scheduled for August 9, 2020. (AP PhotoSergei Grits)

Belarusians wearing face masks to protect against coronavirus, attend a meeting in support of Svetlana Tikhanovskaya, candidate for the presidential elections in Minsk, Belarus, Thursday, July 30, 2020. Opposition candidate Svetlana Tikhanovskaya said she would not cancel any campaign rallies because of the stricter security. Thousands of Tikhanovskaya's supporters gathered Thursday at her campaign rally in Minsk, and many participants dismissed the official claims of a Russian subversion plot as a sham. The presidential election in Belarus is scheduled for August 9, 2020. (AP PhotoSergei Grits)

The Wagner company is linked to Yevgeny Prigozhin, a Russian businessman who was indicted in the United States for meddling in the 2016 U.S. presidential election. The firm has allegedly deployed hundreds of military contractors to eastern Ukraine, Syria and Libya.

Many observers have pointed out that Belarus long has provided a transit corridor for sensitive Russian operations abroad, alleging that Lukashenko now is trying to use a routine Russian deployment for his own political gain.

Throughout his 26 years as president, Lukashenko, a former collective farm director, has relied on Russian subsidies to keep the nation’s Soviet-style economy running but resisted Moscow’s push for closer integration of the neighboring nations. He frequently accused the Kremlin of harboring plans to incorporate Belarus and vowed to resist them.

Belarus President Alexander Lukashenko, right, inspects police vehicles as he visits the Belarusian Interior Ministry special forces base in Minsk, Belarus, Tuesday, July 28, 2020. The presidential election in Belarus is scheduled for Aug. 9, 2020. (Nikolai PetrovBelTA Pool Photo via AP)

Belarus President Alexander Lukashenko, right, inspects police vehicles as he visits the Belarusian Interior Ministry special forces base in Minsk, Belarus, Tuesday, July 28, 2020. The presidential election in Belarus is scheduled for Aug. 9, 2020. (Nikolai PetrovBelTA Pool Photo via AP)

The 65-year-old Belarusian leader is campaigning to retain his post amid a wave of opposition protests driven by public weariness with his iron-fisted rule and a bruising economic damage from the coronavirus pandemic.

Election officials have denied candidate registration to his two main challengers, one of whom was jailed. The other fled to Russia with his children.

The opposition has united behind a third candidate, Svetlana Tikhanovskaya, the wife of a jailed opposition blogger. On Thursday, Belarus’ Investigative Committee linked the detained Russians to her husband as part of a criminal investigation into alleged preparations for staging “mass riots.”

FILE - In this file photo taken on Tuesday, June 30, 2020, Russian President Vladimir Putin, left, and Belarusian President Alexander Lukashenko greet WWII veterans during an opening ceremony of the monument in honour of the World War II Red Army, in the village of Khoroshevo, just outside Rzhev, about 200 kilometers (about 125 miles) northwest of Moscow, Russia. President Alexander Lukashenko is seeking a sixth term in the Aug. 9 election but he has accused Russian forces of interfering with the upcoming vote, although Moscow has denied any involvement. (Mikhail Klimentyev, Sputnik, Kremlin Pool Photo via AP, File)

FILE - In this file photo taken on Tuesday, June 30, 2020, Russian President Vladimir Putin, left, and Belarusian President Alexander Lukashenko greet WWII veterans during an opening ceremony of the monument in honour of the World War II Red Army, in the village of Khoroshevo, just outside Rzhev, about 200 kilometers (about 125 miles) northwest of Moscow, Russia. President Alexander Lukashenko is seeking a sixth term in the Aug. 9 election but he has accused Russian forces of interfering with the upcoming vote, although Moscow has denied any involvement. (Mikhail Klimentyev, Sputnik, Kremlin Pool Photo via AP, File)

Speaking Thursday at a rally in Minsk that drew tens of thousands of people in the biggest demonstration since the presidential campaign's start, Tikhanovskaya dismissed the charges against her husband as a crude fabrication.

Yuras Karmanau in Minsk contributed to this report.