By Pavel Polityuk, Natalia Zinets and Omer Berberoglu LVIV, Ukraine/KYIV (Reuters) -Russian air strikes and artillery fire smashed into buildings in the Ukrainian capital Kyiv on Tuesday as invading forces tightened their grip, while a convoy of civilians headed out of the besieged port city of Mariupol for safer parts. At least five people were killed in the bombardments on Kyiv on the 20th day of the Russian assault, authorities said. Buildings were set ablaze and people were buried under the rubble. Despite the danger, the leaders of Poland, Slovenia and the Czech Republic arrived in the ci…