Human rights activists this week stepped up their criticism of President Obama's use of armed drones to kill terrorism suspects, after The New York Times reported that the Obama administration is drawing up a rule book spelling out when such targeted assassinations are justified. The U.S. military and intelligence agencies are increasingly relying on remotely controlled aircraft for surveillance and for taking out enemies, and other nations are scrambling to catch up by putting their own drones in the air. Here, a look at this 21st century arms race, by the numbers:

Estimated minimum number of people who have been killed in attacks by armed American unmanned aircraft since President Obama took office

Suspected Taliban and al Qaeda leaders who have been killed by U.S. drones in Pakistan since 2004

U.S. airstrike in Pakistan in 2004

U.S. airstrikes in Pakistan in 2008

U.S. drone strikes in Pakistan in 2009. That was President Obama's first year in office, and the year when these attacks peaked.

Unmanned aerial vehicle strikes the U.S. has launched in Afghanistan since 2009

U.S. drone strikes in Yemen since February, when The New York Times reported that the U.S. was aiming to kill two dozen al Qaeda leaders in the country. In the same period, the local al Qaeda franchise — al Qaeda in the Arabian Peninsula — has grown from "several hundred" to "a few thousand members."

New models of sophisticated drones unveiled by Chinese companies at the Zhuhai air show in southeastern China last November. One presentation even featured an animation showing an armed drone attacking an American aircraft carrier.

Range, in miles, of a Shahed-129, Iran's newest drone

Miles between Tehran and Tel Aviv

Drones in the U.S. arsenal today

$36.9 billion
Amount the U.S. plans to invest to increase the size of its surveillance- and combat-drone force by 35 percent over the next eight years

$94 billion
Estimated global spending on research and purchase of drones over the next decade

$9 billion
Chunk of that money that will be spent on remotely piloted combat aircraft

Countries that already have unmanned aerial vehicles. Most of them are surveillance drones, although many could be fitted with missiles or bombs fairly easily. "I think of where the airplane was at the start of World War I: At first it was unarmed and limited to a handful of countries," P. W. Singer of the Brookings Institution, author of the 2009 robotic combat primer Wired for War, tells The New York Times. "Then it was armed and everywhere. That is the path we're on."

Sources: Huffington Post, Long War Journal, New York Times (2), ProPublica