A man claiming to have an explosive device has barricaded himself in a post office in the Ukrainian city of Kharkov. The perpetrator held around a dozen people, including children, hostage for several hours.

An unnamed man was detained after he held people hostage for around six hours Saturday. None of the captives were injured, authorities said, adding that psychologists are now counseling them.

“A man inside the Ukrainian post [branch] announced that he is wearing an explosive device, which he is ready to use,” National Police spokesman Yaroslav Trakalo told Ukrainskaya Pravda earlier in the day.

В #Харьков на ул.Шевченко возле ТЦ "Рост" неизвестный захватил отделение "Укрпочты", внутри есть люди. Фото - Евгений Руденко pic.twitter.com/4Mp0k4mSYg — Корень в квадрате (@korenvkvadrate) December 30, 2017

Eleven people have reportedly been taken hostage, Interfax Ukraine reported, citing the local head of the national police. “There are nine adults and two children” inside the post office, Oleg Bekh told the news agency.

Hours later, five hostages – two children and three women – were released, according to the Ukrainian president’s press secretary, Svyatoslav Tsegolko. Six more people remained in the attacker’s hands.

Dozens of emergency service vehicles, including police cars and ambulances, were present at the scene. The area on Shevchenko Street where the post office is located has been sealed off.

The emergency call was received from a postal worker around 2:40pm local time, the police spokesman said. According to the caller, the suspected attacker entered the building wearing a mask.

К месту захвата заложников в #харьков’ском почтовом отделении прибыл полицейский спецназ КОРД.Фото: Є.Руденко / Укр.правда pic.twitter.com/JZ1Ny9xJP5 — StudMedia (@StudMedia) 30 декабря 2017 г.

Contact has been established with the suspected attacker, the police official told 112 Ukraine TV. Negotiators have talked to the man through post office customers’ mobile phones, he said, adding that the demands were uncertain. The man claimed he had both “explosives and weapons,” but did not specify further, Bekh said.

The man initially planned to rob the post office, an adviser to Ukraine’s interior minister, deputy Anton Gerashchenko, wrote on Facebook. The official explained that some of the customers managed to call the police, who arrived at the scene within minutes. “The robber realized that there was no way he could flee the crime scene with his loot, and decided to announce he is taking everyone hostage,” Gerashchenko wrote.

Speaking to 112 Ukraine TV, the deputy said that should the attacker refuse to turn himself in, the building might be taken by force.

An unverified video allegedly showing the attacker inside the premises has emerged online. In the footage, a man in dark clothes and a black mask is seen with some unidentifiable items attached to his body. The wired-up man is heard saying that "a blast will go off... When my hand gets tired, I'll switch [the device] to a special pedal."