The death toll among Afghanistan's security forces since 2015 is close to 29,000, Afghan President Ashraf Ghani has revealed, providing a figure far higher than anything previously acknowledged by the government.

Ghani disclosed the figure during a video appearance this week before the Johns Hopkins School of Advanced International Studies in Washington, a school where he once taught as a professor of anthropology.

Ghani said that since the start of 2015, when Afghan police and army units took over primary responsibility for Afghanistan's security after a partial withdrawal of NATO troops, "28,529 of our security forces have lost their lives and become martyrs for our freedom."

In the same time frame, 58 Americans were killed, he said.

"I would like to salute the patriotism of the Afghan security forces, every single one of whom is a volunteer," Ghani said.

"We have no conscription, nobody is forced, and if there was not a patriotic impulse, I don't think that people would sacrifice their lives for a pay of $200."

The Afghan government has previously provided casualty figures only for individual incidents of clashes between Afghan troops and militants, but the figures it has provided have suggested the overall death toll might be large.

In one incident alone last year, the government reported that a Taliban attack on a military compound in Balkh Province left more than 130 army personnel dead.

The Afghan government and U.S. military have previously been reluctant to disclose the full extent of Afghan soldiers killed, apparently out of concern that acknowledging the large death toll would undermine confidence and underscore the country's fragile security situation.

Previous estimates given for the total Afghan security forces killed in 2015 were around 5,000, with another 15,000 wounded.

Since that time, estimates have suggested the death toll was accelerating despite claims of progress in the war against the Taliban by both U.S. and Afghan leaders.

A year ago, the U.S. commander in Afghanistan General John Nicholson said casualty rates among Afghan security forces were starting to drop as the United States increased its air strikes against the Taliban and Afghan forces adopted military reforms.

But a recent report from the U.S. Special Inspector General for Afghanistan Reconstruction cited the NATO mission in Kabul as saying the toll between May and October was worse than ever for Afghan forces, without providing specific figures.

U.S. Defense Secretary Jim Mattis this month said more than 1,000 "Afghan lads" were killed or wounded just in August and September.

After more than 17 years of war, the administration of U.S. President Donald Trump has stepped up efforts to settle the conflict peacefully.

U.S. special envoy for Afghanistan Zalmay Khalilzad was appointed to start a peace process with the Taliban. He met with Ghani on November 10 after meeting with a Taliban delegation in Doha in October.

With reporting by AFP