Ukrainians boarded buses to flee the besieged eastern city of Sumy on Tuesday (Mar 8), the first evacuation from a Ukrainian city through a humanitarian corridor agreed with Russia. Ukraine said a separate convoy of 30 buses was also headed to Mariupol to evacuate residents from that southern port, which has been encircled without food, water, power or heat and subjected to relentless bombardment for a week.

The United Nations said the number of refugees who have fled Ukraine has surged past 2 million, describing the flight as one of the fastest exoduses in modern times. The economic impact of a conflict involving the world's top oil and gas exporter and two of its biggest grain and metals producers, was also intensifying on Tuesday, fuelling concerns it could derail global recovery from the coronavirus pandemic.

US gasoline pump prices hit a record high, London trade in industrial metal nickel had to be suspended after prices more than doubled in a matter of hours, and Britain's Shell said it was halting all purchases of Russian oil, apologising for buying a shipment last week. The evacuations from Sumy to the city of Poltava further west began hours after a Russian air strike in Sumy,

which local authorities said killed 21 people. Reuters could not verify that report. "We have already started the evacuation of civilians from Sumy to Poltava, including foreign students," Ukraine's foreign ministry said in a tweet. We call on Russia to uphold its ceasefire commitment, to refrain from activities that endanger the lives of people and to allow the delivery of humanitarian aid."

Dmytro Zhyvytsky, the governor of the Sumy region, said in a video statement that a second column of civilians would leave Sumy at around 11am GMT. A short video clip released by presidential advisor Kyrylo Tymoshenko showed a red bus with some civilians on board.

Residents were also leaving the town of Irpin, a frontline Kyiv suburb where Reuters journalists had filmed families fleeing for their lives under fierce bombardment on Sunday. Residents ran with their young children in strollers or cradling babies in arms, while others carried pets and plastic bags of belongings. "The city is almost ruined, and the district where I'm living, it's like there are no houses which were not bombed,"

said one young mother, holding a baby beneath a blanket, while her daughter stood by her side. Moscow describes its actions in Ukraine as a "special operation" to disarm its neighbour and unseat leaders it calls neo-Nazis. Ukraine and its Western allies call this a baseless pretext for an invasion to conquer a country of 44 million people.

Western sanctions have cut off Russia from international trade and financial markets to a degree never before imposed on such a big economy, while fighting in Ukraine's south has largely blocked its exports. Russia's Interfax news agency said Moscow was opening humanitarian corridors on Tuesday to allow people to leave five Ukrainian cities: Sumy, Maripol, Chernihiv, Kharkiv and the capital Kyiv.

Ukraine has rejected Russian proposals for Kharkiv and Kyiv which would see evacuees sent to Russia itself or its ally Belarus. Earlier attempts to evacuate residents from Mariupol failed on Saturday and Sunday, with each side accusing the other of continuing to fire. United Nations was discussing humanitarian access with both sides, with a delegation in Moscow since Monday,

a spokesperson for its humanitarian affairs office said. DISASTER-- Ukraine said the pace of Russia's advances had slowed on Tuesday. Its defence ministry said Russian Major General Vitaly Gerasimov, first deputy commander of Russia's 41st army, had been killed on Monday, the second Russian major general killed since the invasion began.

Russia's defence ministry could not be immediately reached for comment. "The tempo of the enemy's advance has slowed considerably, and in certain directions where they were advancing it has practically stopped," Ukrainian presidential adviser Oleksiy Arestovych told a televised briefing on Tuesday. "The forces that continue to advance, advance in small forces."

At the Medyka border crossing, east of Przemysl in Poland, where refugees have been arriving by foot and by car, a line of waiting vehicles stretched about 6km with waiting times on the Ukrainian side of the border running at 20 hours, two women told Reuters after they made it across. Western countries say Russia's initial battle plan for a rapid strike to topple the Kyiv government failed in the early days of the war, and Moscow has adjusted tactics for longer sieges of cities.

The main assault force heading towards Kyiv has been stuck on a road north of the capital. But to the south, Russia has made more progress along the Black and Azov Sea coasts. Fears of an energy war between Russia and the West have grown this week, after Washington pushed its allies to ban Russian oil imports, so far exempted from sanctions.

The United States is not a major buyer of Russian energy, but Russia supplies 40 per cent of Europe's gas. Russian Deputy Prime Minister Alexander Novak warned on Monday Russia could halt deliveries to Germany in retaliation for Berlin suspending a new pipeline. Within Russia, the war has led to a severe new crackdown on dissent,

with the last remaining independent media largely shut down last week and foreign broadcasters banned. Many foreign news organisations have suspended reporting after a new law was enacted imposing long jail terms for reporting deemed to discredit the military. The top UN human rights official, Michelle Bachelet, said 12,700 people in Russia had already been detained at anti-war demonstrations.