Dozens of passengers, mainly women and children, were killed in western Afghanistan early Wednesday when the bus they were travelling in hit a roadside bomb, officials said.

"A passenger bus travelling on the Kandahar-Herat highway hit a Taliban roadside bomb. So far at least 28 killed, 10 wounded," said Muhibullah Muhib, the spokesman for Farah province.

All were civilians, mostly women and children, he said. Farooq Barakzai, a spokesman for Farah's governor, confirmed the toll but warned it may rise higher.

A man holds the body of a child killed by the bomb.

There was no immediate confirmation from the Taliban that it was behind the attack.

It came one day after the United Nations said civilians are being killed and wounded at a "shocking" level in Afghanistan's war, despite a push to end the nearly 18-year-old conflict.

Casualties have dropped 27 per cent in the first half of 2019 compared to the same period last year, which was a record, but nonetheless 1,366 civilians were killed and another 2,446 injured. The UN branded efforts to reduce the violence "insufficient".

It also said that US and pro-government forces caused more civilian deaths than the Taliban and other insurgent groups for the second quarter running.

Child casualties represented almost one-third of the overall total of civilian casualties.

The bloodshed comes amid a months-long, US-led push to forge a peace deal with the Taliban that would see foreign forces quit the country in return for various security guarantees.