AVR bit manipulation compilation in GCC: can not get SEI instead of read-modify-write
I found it:

Citation from original datasheet (page 20):

I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and CBI instructions

So only first 32 registers are bit-accessible. TIMSK has address 0x39 - so it's pretty high above the range :(