The number of U.S. drone strikes in Pakistan plunged last year amid growing care to avoid civilian deaths, but the death toll in neighboring Afghanistan continues to rise, the United Nations' special investigator on counterterrorism said Wednesday.

Ben Emmerson says that for the first time in nine years no civilians were reported killed in 2013 in Pakistan's Federally Administered Tribal Regions, or FATA, a semi-autonomous area along the 1,600-mile Afghanistan-Pakistan border where militant groups operate.

Emmerson, a British lawyer who reports to the 47-nation Human Rights Council and the U.N. General Assembly, told reporters in Geneva there has been “a very significant de-escalation” of U.S. armeddrone use in Pakistan's FATA region, down to 27 strikes last year from a peak of 128 in 2010.

But he said the picture is uglier in Afghanistan, where drone strikes and civilian deaths resulting from their use are intensifying. In Afghanistan, he said, the number of civilian casualties from drone strikes last year rose to 45 dead and 14 injured, triple the rate experienced in 2012. 

Before his press conference, Emmerson reported to the Geneva-based council on the civilian toll from drone attacks, chiefly carried out by U.S. forces, and called for more independent investigation.

He examined about 30 drone strikes involving civilian casualties committed by American, British and Israeli forces in Afghanistan, Gaza, Pakistan, Somalia and Yemen.

He said the U.S. military had reduced the frequency of drone attacks, and limited them to high-value targets, because of growing criticism in Pakistan and abroad.