By Natalia Zinets (Reuters) -An aide to the mayor of Mariupol said on Friday the besieged southern Ukrainian city remained closed for anyone trying to enter and was “very dangerous” for anyone trying to leave. Petro Andryushchenko said Russian forces had since Thursday been preventing even the smallest amount of humanitarian supplies reaching trapped residents, making clear a planned “humanitarian corridor” had not been opened. Tens of thousands of civilians have been trapped for weeks with scant food, water and other supplies in the city that was once home to 400,000 people but has been devas…