An explosion on Wednesday hit a bus travelling on a highway between two key cities in western Afghanistan, killing at least 34 passengers, including children, and injuring 17, officials said.

Security has been deteriorating across Afghanistan, with the Taliban and Daesh fighters mounting near-daily attacks on Afghan forces, government employees and civilians.

Medical staff at a hospital in Herat is treating an injured Afghan man on Wednesday. Hoshang Hashimi/AFP

The blast on the highway linking the provincial capitals of Herat and Kandahar took place in the Ab Khorma area of Farah province, said provincial police spokesman Mohibullah Mohib.

"The bomb was freshly planted by the Taliban insurgents to target Afghan and foreign security forces," he said.

Taliban officials were not immediately available to comment on the statement and there was no immediate claim of responsibility for the attack.

Government and aid officials say the numbers of those being killed and maimed is rising because of explosives newly planted by the Taliban, who now control more territory than at any point since their ouster nearly 18 years ago.

Wednesday's blast comes ahead of presidential elections next month, when U.S. officials and Taliban representatives are also due to resume talks to agree a timetable for the withdrawal of foreign forces in exchange for security guarantees.

US soldiers killed

An Afghan soldier was responsible for the killing of two American troops a day earlier, an official told the media on Tuesday, in what appears to be the latest example of an insider attack.

The US military on Monday said two of its troops had been killed in action in Afghanistan, but did not provide any additional details, pending notification of next of kin.

An Army carry team moves a transfer case containing the remains of Spc. Michael I. Nance at Dover Air Force Base, Del., on Wednesday.

Mohammad Qasam, a deputy police chief in Kandahar province in southern Afghanistan, said the attack took place at an Afghan army base during a visit by US forces.

"Initial information was two US forces have been killed and one more wounded," Qasam said.

US Forces-Afghanistan declined to comment.

In a statement to AFP, Taliban spokesman Zabihullah Mujahid said an Afghan soldier, not an infiltrator, had killed the two.

"A patriotic Afghan soldier opened fire on them and killed them" in the attack in Kandahar, Mujahid said.

An Afghan man holds the dead body of a child killed in a roadside bomb on the Kandahar-Herat highway.

Bahir Ahmadi, the Kandahar governor's spokesman, also said the incident occurred at an Afghan army base.

On July 17, an Afghan army soldier shot and killed an influential Afghan army colonel while he was conducting a security assessment in Ghazni province.

The Taliban claimed responsibility for that killing.

Insider attacks, sometimes referred to as "green on green," are a constant threat in Afghanistan for international and Afghan forces alike.

Agencies