The Afghan Taliban have announced the start of their warm-weather fighting season, an annual declaration marking the launch of a violent summer.

In an email to media organizations Tuesday, the militants said the so-called "spring offensive" had begun at 5 a.m.

The campaign, expected this year to be long and bloody, is called "Operation Omari" in honor of Taliban founder, Mullah Mohammad Omar, who died three years ago, the announcement says.

It says waging jihad against the American invaders is a holy obligation. The war is in its 15th year.

The group has been riven by infighting since Mullah Omar's death became public last summer. New leader Mullah Akhtar Mansoor had run the insurgency in his name for more than two years.

More than 11,000 civilians were killed in 2015.