By Anton Zverev DONETSK Ukraine (Reuters) - The Ukrainian city of Donetsk was rocked by blasts on Saturday, even as government forces and pro-Russian separatists prepared to create a buffer zone to separate the warring sides. A memorandum signed early on Saturday calls for the withdrawal of heavy weapons, including artillery, and all foreign fighters from a 30 kilometer-wide buffer zone. A Reuters correspondent in Donetsk, the main industrial hub in Ukraine's turbulent east, said several powerful explosions were heard in the morning. ...