In a speech Thursday, Jonathan Evans said his officers are engaged in an "intense struggle" on both fronts, The Daily Telegraph reported.
It is only "a matter of time" before extremists based in Somalia strike at Britain, the director general of the security service added.
Evans cited a "persistent rise in terrorist activity and ambition" in Northern Ireland using increasingly sophisticated weapons over the past three years. He said dissident Republican groups "might seek to extend their attacks to Great Britain as violent Republican groups have traditionally done."
Evans appealed to the government not to drop control orders, under which terrorist suspects are electronically tagged and put under house arrest. Nine suspects are currently subject to them.
The Liberal Democrats had promised to scrap the measure and it is now being considered under a wider review of counter-terrorism legislation.
Government sources have told The Daily Telegraph control orders are likely to be retained in some form.
Duggar sisters unveil Christian dating rules in new book
Beautician charged with giving client fatal silicone butt injection