(NECN/NBC News: Tracie Potts) - The Taliban in Afghanistan is vowing revenge Monday morning for the deaths of 16 civilians allegedly shot and killed by a U.S. soldier.
Most of the victims of the brutal slayings were women and children.
While President Barack Obama has apologized, this is a serious blow to the already-strained relationship between the U.S. and Afghanistan.
Some in Afghanistan are calling for protests on Monday after what their government - and lawmakers in Washington - call a senseless act of violence.
Afghanistan's president called it terrorism.
A U.S. soldier shot and killed nine children, three women and four men, just 500 yards from a U.S. military base in southern Afghanistan. He's in custody.
"These incidents and in particular the short distance between the incidents that we've seen in the recent weeks are a burden and are of concern for ISAF," said Gen. Carsten Jacobsen, ISAF spokesman.
"NATO is sitting in a tinderbox and this could be the spark that unravels our presence in Afghanistan," warned retired U.S. Army Gen. Barry McCaffrey.
President Obama called Afghan President Hamid Karzai on Sunday to apologize, saying the U.S. will investigate and the hold the alleged shooter responsible.
Karzai, in a statement, called it an intentional act that will never be forgotten, one month after accidental burnings of the Qu'ran sparked deadly protests in the country.
"Unfortunately, these things happen in war. You had an Israeli soldier kill worshipers by the Dome of the Rock mosque," said Republican Sen. Lindsey Graham of South Carolina. "You just have to push through these things."
In Washington, lawmakers say this latest incident underscores why the U.S. needs to get out of Afghanistan.
"Our timetable is pretty good," said Democrat Sen. Harry Reid of Nevada on CNN's State of the Union. "We're moving out, as the President said. I think that's the right thing to do."
However, some lawmakers don't want troops to leave the country too quickly.
"We should not forget the attacks on the United States of America on 9/11 originated in Afghanistan," warned Republican Sen. John McCain.