Sixteen ships loaded with grain were due to depart from the Ukrainian port of Odessa as tensions escalate again after a missile attack killed dozens of Ukrainian prisoners of war. The office of Ukrainian President Volodymyr Zelensky said on Saturday that the departure of the ships was imminent. Al Jazeera’s John Hedren, reporting from Odessa, said 25 million tonnes of grain was to be sent to Africa, the Middle East and other parts of the world as part of a UN-backed grain export deal signed between Moscow and Kiev on July. 22, “They have mapped out a safe channel for these ships to leave Ukraine,” Hedren said, adding Zelensky visited the port of Chernomorsk in the Odesa region on Friday to watch crews preparing to export grain. “The first ship, the first ship is being loaded from the beginning of the war,” Zelensky told reporters. Despite the Ukrainian leader’s hopeful words, no ships have yet left Ukrainian ports. “One of the issues they may be dealing with is that these waters are mined and people have been injured and killed,” Hedren said. The passage of the ships “also requires a certain degree of trust between the warring nations,” he added, which was further undermined on Friday by an attack on a detention center in Olenivka, in the separatist-held region of Donetsk. Russia accused Kyiv of hitting the prison with the US HIMARS missile system, killing dozens of inmates, including some whom Kyiv considered war heroes for defending the Mariupol Azovstal steel plant. Ukrainian forces denied carrying out the attack and said they refrained from hitting civilian infrastructure in compliance with international law. “This is a deliberate war crime on the part of Russia, a deliberate killing of Ukrainian prisoners of war,” Zelensky said. Hedren said the latest report said 50 inmates were killed, up from the 40 reported on Friday. “Both sides have called for an international investigation to find out what caused the explosion,” Al Jazeera’s correspondent added. “One of the things they will look at is whether there was an external strike or whether it was internal – that’s what the Ukrainian side claims.” The tensions could hamper the export target of about four to five large bulk trucks a day to transport grain from ports to millions of people in Africa, the Middle East and Asia, who are already facing food shortages and, in some cases, famine . UN agencies such as the World Food Program have already arranged to charter much of the grain for emergency humanitarian needs.