Afghan security forces were killed at a "shockingly high" rate during what has historically been a winter lull in fighting against the Taliban, according to a report by a US watchdog.

The US government's Special Inspector General for Afghanistan Reconstruction (SIGAR) says 807 troops from the Afghan National Defence and Security Forces (ANDSF) died between January 1 and February 24.

"Afghanistan remains in the grip of a deadly war. Casualties suffered by [ANDSF] in the fight against the Taliban and other insurgents continue to be shockingly high," says the report, released on Monday.

Levels of violence have traditionally dipped over Afghanistan's cold winter months, but this year the Taliban continued to battle government forces, most notably in an April 19 attack on a military base outside the northern city of Mazar-i-Sharif.

The massacre saw fighters armed with guns and suicide bombs slaughter at least 144 recruits, a US official told AFP news agency, though multiple sources have claimed the toll was higher still.

The Afghan Taliban launched their self-styled spring offensive on Friday, heralding fresh fighting the group said would include "conventional attacks, guerrilla warfare, complex martyrdom attacks [and] insider attacks".

Afghan police and army units took over the task of providing security for the country from NATO in 2015.

According to SIGAR, 6,785 Afghan soldiers and police officers were killed between January 1 and November 12, 2016, with another 11,777 wounded.

Al Jazeera's Rob McBride, reporting from Kabul, said "people here know all too well that the situation is bad" and the report's findings were no surprise in Afghanistan.

He said "it was only a few days ago that the Taliban announced the start of their spring offensive, so we can expect even more attacks from them".

'Contested' territory

The Afghan government has not provided US Forces-Afghanistan with data for the last seven weeks of 2016, but even the partial numbers showed an increase of about 35 percent from all of 2015, when some 5,000 security forces were killed.

The SIGAR report also found that both Afghan government and anti-government groups slightly increased the amount of territory they held, as the number of areas previously considered "contested" dropped.

Afghan forces now control 59.7 percent of the country, up from 57.2 percent the previous quarter.

The Taliban and other armed groups, meanwhile, saw their areas of control or influence increase slightly from about 10 percent to 11.1 percent.

The US Congress created SIGAR to provide oversight into how the more than $100bn appropriated for Afghan reconstruction has been spent since 2002.

Part of its remit is to compile quarterly reports providing snapshots of the country's progress, or lack thereof, and highlight ongoing challenges to the security situation.

The SIGAR report also points to a UN tally showing that civilian casualties in 2016 were the highest since the United Nations Assistance Mission in Afghanistan began recording them in 2009.

That count found conflict-related civilian casualties in Afghanistan rose to 11,418 in 2016, including 3,498 killed and 7,920 wounded.