Two U.S. service members have been killed in action in Afghanistan in what appears to be an insider attack.

The NATO-led Resolute Support Mission said Monday that two Americans are dead, withholding additional information pending notification of their families. A U.S. official later confirmed the deaths were the result of a so-called "green on blue" attack, during which an Afghan service member or an attacker wearing an Afghan uniform, fires on U.S. or allied forces.

The initial U.S. assessment followed claims by Taliban spokesman Zabihullah Mujahid on Twitter that the Americans died when an Afghan soldier turned his gun on them in a military camp in Afghanistan's southern Kandahar province.

#خـــــبر:

امروز در داخل کمپ (بیس) تناوچه ولسوالی شاولیکوت ولایت قندهار یک عسکر با احساس افغان بر عساکر امریکایی از نزدیکی آتش کشود.

۴ عسکر اشغالگر امریکایی به هلاکت رسیدند، ۲ تن زخم برداشتند.

در فیرهای متقابل عسکر با احساس نیز زخمی گردید.

احمدی — Zabihullah (..ذبـــــیح الله م ) (@Zabehulah_M33) July 29, 2019

The Taliban also claimed at least one other soldier was killed and that several others were wounded, though those claims could not be confirmed.

With the two confirmed deaths, at least 14 U.S. service members have been killed in Afghanistan this year, according to a tally by the website iCasualties.org. Thirteen U.S. service members were killed in Afghanistan in 2018.

Talks with Taliban

The latest deaths also come as the U.S. has been engaging the Taliban in talks, seeking a peaceful resolution to the 18-year war in Afghanistan.

FILE - The Twitter page of Taliban spokesman Zabihullah Mujahid is pictured on a computer monitor in the newsroom at Maiwand TV station in Kabul, Feb. 6, 2019.

Taliban negotiators have insisted any peace deal include the withdrawal of U.S. forces, and those of U.S. allies, from Afghanistan. The Taliban have also refused to hold direct talks with the government of Afghan President Ashraf Ghani, saying such talks can only take place once the U.S. leaves.

For its part, Washington is seeking assurances from the Taliban that Afghanistan will never again be used as a base to launch terror attacks against the U.S.

Yet despite the talks, attacks by either the Taliban or the Islamic State terror group have continued to plague much of Afghanistan.

An attack Sunday in Kabul that targeted the office of Amrullah Saleh, Ghani's choice for vice president in elections set for September, killed at least 20 people and wounded another 50, officials said.

No group has claimed responsibility for the bombing.

FILE - Afghan security personnel stand guard after the offices of Amrullah Saleh was targeted in a deadly attack, in Kabul, Afghanistan, July 28, 2019.

In comments in Washington Monday, John Sopko, the U.S. special inspector general for Afghanistan reconstruction (SIGAR), raised concerns that despite Washington's efforts, Afghan security forces may not be capable of sustaining peace, even if a deal with the Taliban can be reached.

"Afghan security forces cannot survive without external donor support, both financial and technical," Sopko said.

"Problems don't miraculously disappear. We, and other oversight bodies, have identified problems that affected reconstruction. And some of these problems could affect lasting peace," he said.

U.S., NATO efforts

SIGAR estimates the U.S. alone has spent about $18 billion to equip Afghanistan's security forces, buying more than 600,000 weapons, 70,000 vehicles and more than 200 aircraft.

But its audits have found that U.S. and NATO efforts have often been unorganized, with Afghan forces suffering as a result.

Specifically, SIGAR blamed an eight-year lag in shifting Afghan forces to the latest standardized NATO weaponry for an ammunition shortage that left them unable to repel a Taliban attack in Ghazni province in 2018.

FILE - A member of the Afghan security forces prepares to fire an RPG during a battle with insurgents in Kabul, Afghanistan, Aug. 16, 2018.

Sopko called a lack of armored ambulances for Afghan security forces — there are just 38 — another "absurd example" of missed opportunities to better support Afghan forces.

SIGAR officials also warned the performance of Afghan forces has tended to rise and fall in sync with the U.S. presence in the country.

""It's like a shark tooth," James Cunningham, SIGAR's security sector project lead, said Monday. "The shark tooth is really based on our [U.S. military] deployment cycle."

Despite such concerns, both U.S. President Donald Trump and other top officials have said the U.S. intended to reduce the number of troops in Afghanistan.

"We want to reduce what is, for us, tens of billions of dollars a year in expenditures and enormous risk to your kids and your grandkids who are fighting for America," U.S. Secretary of State Mike Pompeo told an audience Monday in Washington, adding he expects more U.S. troops to come home before the next U.S. presidential election in November 2020.

"That's my directive from the president," Pompeo said.