Green Change quotes a Glenn Greenwald article that states that the Obama administration has compiled a list people, including US citizens, which president Obama has authorized the military and intelligence services to kill:

Just think about this for a minute.  Barack Obama, like George Bush
before him, has claimed the authority to order American citizens
murdered based solely on the unverified, uncharged, unchecked claim
that they are associated with Terrorism and pose "a continuing and
imminent threat to U.S. persons and interests."  They're entitled to no
charges, no trial, no ability to contest the accusations.  Amazingly,
the Bush administration's policy of merely imprisoning foreign
nationals (along with a couple of American citizens) without charges —
based solely on the President's claim that they were Terrorists —
produced intense controversy for years.  That, one will recall, was a
grave assault on the Constitution.  Shouldn't Obama's policy of
ordering American citizens assassinated without any due process or
checks of any kind — not imprisoned, but killed — produce at least as
much controversy?

Obviously, if U.S. forces are
fighting on an actual battlefield, then they (like everyone else) have
the right to kill combatants actively fighting against them, including
American citizens.  That's just the essence of war.  That's why it's
permissible to kill a combatant engaged on a real battlefield in a war
zone but not, say, torture them once they're captured and helplessly
detained.  But combat is not what we're talking about here.  The people
on this "hit list" are likely to be killed while at home, sleeping in
their bed, driving in a car with friends or family, or engaged in a
whole array of other activities.  More critically still, the Obama
administration — like the Bush administration before it — defines the "battlefield" as the entire world. 
So the President claims the power to order U.S. citizens killed
anywhere in the world, while engaged even in the most benign activities
carried out far away from any actual battlefield, based solely on his
say-so and with no judicial oversight or other checks.  That's quite a
power for an American President to claim for himself.

Greenwald lays it out pretty well and it is worth the read.