Donetsk, Ukraine - Pro-Russian rebels cemented their hold Friday on a long-disputed airport ceded by Ukrainian troops during an upsurge in clashes that killed nearly 50 people and punctured Europe's latest push for peace in the nine-month war.The deadliest day of fighting since the signing of an increasingly irrelevant September truce also saw Moscow and Kiev on Thursday trade bitter blame for a trolleybus shelling in the rebel stronghold of Donetsk that killed 13 mostly elderly passengers.Moscow called the incident a "crime against humanity" orchestrated by a pro-Western government whose rise to power 11 months ago infuriated the Kremlin and prompted separatists to launch a revolt across the Russian-speaking industrial east.Kiev for its part blamed the bus attack on "Russian terrorists" while monitors from the Organisation for Security and Co-operation in Europe who raced to the site said all they could say for certain was that "the weapon used was most likely either a mortar or an artillery piece".Several bloodied bodiesStunned residents in the eastern city gathered around the shredded remains of the bus and inspected with horror several bloodied bodies that remained sprawled in their seats hours after the early morning attack.Kiev suffered its biggest psychological blow on the bloody day when a small unit of Ukrainian paratroopers was forced to abandon its 242-day defence of Donetsk's once-gleaming but now ruined international airport.A clam settled early on Friday over the shattered residential districts near the site after six days of some of the most intense rocket and mortar fire exchanges of the entire conflict.The hub - long stripped of its strategic importance by heavy shelling - had become the symbolic prize of the conflict and had seen some of the heaviest fighting.'They have to be killed'Rebels captured about 20 soldiers in the last hours of battle and paraded them in front of jeering locals who pelted the handcuffed men with snowballs and glass."They have to be punished, like Saddam Hussein. They are killers. They killed our children," a pensioner who identified herself only as Zina told AFP.Kiev and rebel authorities said attacks across the separatist regions of Donetsk and Lugansk on Thursday killed 10 soldiers and some 35 civilians in addition to those who died in the bus attack.Ukrainian President Petro Poroshenko defiantly vowed stand up to Russia's "aggression" and respond forcefully to future rebel attacks."We have pulled up extra reserves, and if the enemy does not want to respect the ceasefire, if the enemy does not want to end the suffering of civilians... we will be ready to hit them in the teeth," he told his top generals during an emergency meeting.Toll hits 5 000The violence has threatened to spiral out of control after a December lull that instilled hope in EU leaders that the diplomatically and economically damaging war on the bloc's eastern frontier could finally be drawing to a close.Western diplomats in Kiev linked the past week's spike in attacks to a reported infusion of Russian forces into the war zone.The Kremlin calls such claims part of a Western propaganda campaign design to cut short President Vladimir Putin's 15-year rule.Yet Moscow concedes that the militias have recently gained more ground than allowed under the September truce terms.This advance comes as negotiators come closer to defining the confines of areas that will remain under temporary rebel control once the fighting ends.Many in Kiev fear that the demarcation line will soon turn into an actual border splitting Ukraine from a resource-rich region that will eventually be folded into Russia.The OSCE said on Thursday that the recent escalation had pushed the war's confirmed death toll to more than 5 000. The European security body said another 10 000 people have been wounded and one million more forced to flee their homes.'Russian occupation plan'Thursday's violence came hours after the foreign ministers of Russia and Ukraine met their counterparts from Germany in France for talks designed to salvage the September ceasefire and weapons withdrawal agreements.German Foreign Minister Frank-Walter Steinmeier said the meeting had "tested the patience of all participants".He said Moscow and Kiev both agreed that the fighting must end. But he said not enough was achieved to allow Poroshenko to meet Putin for the signing of a formal peace deal.Russian Foreign Minister Sergei Lavrov emerged from the conference room first and brushed past reporters without a word."The challenge is not Ukraine. The challenge is Russia," Ukrainian Foreign Minister Pavlo Klimkin told CNN.