NATO will hold a ceremony in Kabul later Sunday formally ending its declared war in Afghanistan, officials said, after 13 years of conflict that have left the country in the grip of worsening insurgent violence.

NATO will hold a ceremony in Kabul later Sunday formally ending its declared war in Afghanistan, officials said, after 13 years of conflict that have left the country in the grip of worsening insurgent violence.

The event was arranged in secret due to the threat of Taliban strikes in the Afghan capital, which has been hit by repeated suicide bombings and gun attacks over recent years.

On January 1, the US-led International Security Assistance Force (ISAF) combat mission, which has suffered 3,485 military deaths since 2001, will be replaced by a NATO "training and support" mission.

About 12,500 foreign troops staying in Afghanistan will not be involved in direct fighting, but will assist the Afghan army and police in the battle against the Taliban, who ruled from 1996 until 2001.

When numbers peaked in 2011, about 130,000 troops from 50 nations were part of the NATO military alliance.

"In just a few days, our combat mission in Afghanistan will be over," US President Barack Obama said in his Christmas Day address. "Our longest war will come to a responsible end."

The United Nations says that civilian casualties hit a record high in 2014, jumping by 19 percent with 3,188 civilians killed by the end of November.

Afghan's police and army have also suffered a grim death toll on the battlefield, with fatalities soaring to more than 4,600 in the first 10 months of 2014, far higher than all ISAF deaths since 2001.