A former head of the breakaway ethnic Armenian government in Nagorno-Karabakh was arrested by Azerbaijan on Wednesday as he tried to escape into Armenia as part of an exodus of tens of thousands of people that has triggered a humanitarian crisis.

Ruben Vardanyan, a billionaire banker and philanthropist, headed Karabakh’s separatist government between November 2022 and February 2023.

His wife Veronika Zonabend said on his Telegram channel that he had been arrested while trying to flee as part of a mass departure by ethnic Armenians after Azerbaijan took back control of Karabakh in a lightning offensive last week.

Azerbaijan’s border service said he had been taken to the capital Baku and handed over to other state agencies.

Karabakh is internationally recognised as part of Azerbaijan but populated mostly by ethnic Armenians who broke away in the 1990s in the first of two wars there since the collapse of the Soviet Union.

Azerbaijan, a mainly Muslim country, says it wants to peacefully reintegrate the Armenians and will guarantee their civic rights, including to practise their Christian religion. It says no one is forcing them to leave.

But given the bloody history between the two sides, ethnic Armenians are fleeing in fear, abandoning their homes and boarding cars and trucks that have jammed the snaking mountain road that leads to Armenia.

Karabakh authorities said more than 50,000 had left so far, out of an estimated ethnic Armenian population of 120,000.

Azerbaijan rejects Armenian accusations of ethnic cleansing, but images of tens of thousands of desperate people on the move have provoked widespread international alarm.

The European Union said it was sending more humanitarian aid “in solidarity with those who had no choice but to flee” - a significant change from an earlier statement where it referred to people who had “decided to flee”.

U.N. special rapporteur Morris Tidball-Binz said Azerbaijan must “promptly and independently investigate alleged or suspected violations of the right to life reported in the context of its latest military offensive in Nagorno-Karabakh”.

Germany added its voice to U.S. calls for Azerbaijan to allow international observers into Karabakh.

“What is needed now is transparency, and the eyes and ears of the international community on the spot,” Foreign Minister Annalena Baerbock posted on X, formerly Twitter.

U.S. State Department spokesperson Matthew Miller said Washington would work with allies and partners in the coming days to “flesh out exactly what that mission will look like”.

It was not clear on what grounds Vardanyan, the former Karabakh state minister, had been held, but Azerbaijan has signalled it will seek to prosecute some of the separatists.

“We have accused elements of the criminal regime and we will bring them to justice,” President Ilham Aliyev said last week, without naming anyone or specifying any crime. He described the Karabakh leadership as a “criminal junta” and a “den of poison”.

During his short time in office, Azerbaijan had called Vardanyan an obstacle to peace. He had also fallen out with Armenia’s prime minister over the role of Russian peacekeepers.

His wife Zonabend said she asked for people’s “prayers and support for my husband’s safe release”.

Miller told reporters Washington was aware of Vardanyan’s arrest and was “closely monitoring the situation”.

Tens of thousands have been killed in wars over Karabakh since the 1991 fall of the Soviet Union, of which both Armenia and Azerbaijan were part.

Karabakh authorities said they lost at least 200 people in Azerbaijan’s offensive last week. Baku said on Wednesday that 192 of its soldiers had been killed, and published their names and photographs. More than 50 were young men in their teens.

The mountain road that winds out of Karabakh towards Armenia has been choked for days, with many people sleeping in cars or searching for firewood to keep warm. The journey of just 77 km (48 miles) to the border was taking at least 30 hours.

“I left everything behind. I don’t know what is in store for me. I have nothing. I don’t want anything,” Vera Petrosyan, a 70-year-old retired teacher, told Reuters.

Local authorities said at least 68 people had been killed, 105 were missing and nearly 300 were injured in a huge explosion at a fuel station in Karabakh on Monday. It was unclear what caused it.

Russia said its peacekeeping force in the region had evacuated more than 120 people by helicopter.

Armenia is angry that the Russian peacekeepers, in place since a 44-day war in 2020, did nothing to prevent Azerbaijan from launching its offensive, which swiftly forced the Karabakh leadership to agree to disband and disarm.

With Russia distracted by the war in Ukraine, the crisis has highlighted its waning ability to play the role of security guarantor in the Caucasus region, where Turkey, Iran and the United States are competing with it for influence.

State Department spokesperson Miller told reporters that while Russia has at times facilitated negotiations, which Washington welcomed, “certainly in the last week their role has not been productive in this situation.”