ISLAMABAD - More than 900 Afghan security personnel died in July while battling Taliban insurgents, the commander of U.S. and NATO forces in Afghanistan said Wednesday.

General John Nicholson of NATO’s Resolute Support mission made the revelation during a year in which Taliban fighters have made territorial advances, fighting Afghan forces in more than half of the country's 34 provinces.

In July, "there were over 900 martyrs in the Afghan army and police. This is almost a brigade of soldiers and police, and we should not forget for one minute the sacrifice that they are making even as we meet here today," Nicholson said at a Kabul conference attended by Afghan leaders and foreign diplomats.

According to U.S. military estimates, Afghan forces suffered around 20,000 casualties, including nearly 5,000 dead, during last year’s fighting.

Head of NATO and U.S. forces in Afghanistan, U.S. FILE - U.S. Army Gen. John W. Nicholson heads NATO's Resolute Support mission in Afghanistan. FILE - U.S. Army Gen. John W. Nicholson heads NATO's Resolute Support mission in Afghanistan.

Battling corruption

In his speech, Nicholson emphasized the need to replace "corrupt" leaders in the Afghan security forces and investigate all suspected violations to build trust with the Afghan population and the international community. He also praised the sacrifices of Afghan officers such as Lieutenant Mohammad Akbar of the Special Forces, killed during last week’s attack on Kabul’s American University in Afghanistan.

"You need leaders and officers ready to work harder, change attitudes and practices, be open to new ideas and stop the old ways of doing things," the American commander noted. "Some units are plagued by poor leadership, incompetence and corruption. So, just as we have heroes like Lieutenant Akbar, we also have leaders who are corrupt and need to be replaced."

Nicholson emphasized NATO’s continuing support for Afghan forces "in this hard work" against the Taliban. "But increasing momentum rest[s] on your leadership and on eliminating corruption."

Taliban’s reach grows

The Taliban has extended its insurgent activities to more Afghan regions this year than at any point since the U.S.-led military coalition ousted the Islamist group from power in 2001.

According to the U.S. Special Inspector General for Afghanistan Reconstruction, the Afghan government lost control over 5 percent of its territory to the Taliban in the year's first five months.

The United States contributes around $3 billion of the more than $5 billion the international community provides each year to fund Afghanistan's defense and security forces.

Map locating Taliban controlled districts in Helma Taliban insurgents have been especially aggressive in Afghanistan's Helmand province. Taliban insurgents have been especially aggressive in Afghanistan's Helmand province.

Intense fighting in Helmand

Fighting has been particularly intense in southern Helmand province, the country’s largest and a longtime hub of poppy production. The insurgents have seized control of several districts, with recent clashes taking place not far from the provincial capital, Laskhar Gah.

The Taliban also has made gains in northern and northeastern provinces, including Kunduz, Baghlan and Takhar, near the country’s border with Central Asian countries.

Last week, the insurgents captured an eastern district near the border with Pakistan, opening a new war front and adding to the challenges facing embattled Afghan forces.