The Taliban launched their annual spring offensive on Wednesday, in an apparent rejection of calls for the militants to take up the Afghan government's offer of peace talks.
Operation Al Khandaq -- named after a famous seventh century battle in Medina in which Muslim fighters defeated "infidel" invaders -- will target US forces and "their intelligence agents" as well as their "internal supporters", a Taliban statement said.
The Taliban said the offensive was partly a response to US President Donald Trump's new strategy for Afghanistan announced last August, which gave US forces more leeway to go after insurgents.
The annual spring offensive traditionally marks the start of the so-called fighting season, though this winter the Taliban continued to battle Afghan and US forces.
The group also launched a series of devastating attacks in the Afghan capital Kabul, killing and wounding hundreds of civilians.
Al Khandaq will mainly focus on "crushing, killing and capturing American invaders and their supporters", the Taliban said.
It added the presence of American bases "sabotages all chances of peace" and were key to "prolonging the ongoing war", which began with the US-led intervention in 2001 that overthrew the Taliban regime.
Afghanistan's largest militant group has been under pressure to accept Afghan President Ashraf Ghani's February offer of peace talks, but the statement made no mention of the proposal.
Western and Afghan experts said the Taliban announcement was an apparent rejection of the offer and heralded more intense fighting in the drawn-out war.
"We're in for a hot and busy summer," a foreign diplomat in Kabul told AFP.
Afghan political analyst Ahmad Saeedi said the Taliban appeared to consider America's rejection of the group's own request for direct peace talks with the US in February as leaving them with "no other choice but to fight".
"This year they will try to weaken the (Afghan) government even further. They will try to derail the election process," the Kabul University professor told AFP.
"A weak government would eventually mean forcing the US to talk to them."
Defence ministry spokesman Mohammad Radmanish dismissed the Taliban announcement as "propaganda".
The US-backed Afghan government is under pressure on multiple fronts this year as it prepares to hold long-delayed legislative elections even as its security forces struggle to get the upper hand on the battlefield and prevent civilian casualties.
On Sunday, a suicide bomber blew himself up in a crowd outside a voter registration centre in Kabul, killing 60 people and wounding 129, according to the latest figures from the health ministry.
The Islamic State group claimed responsibility for the bomb, but Western and Afghan officials suspect IS receives assistance from other groups, including the Taliban's Haqqani Network, to carry out attacks.