KABUL (Reuters) - The Taliban announced the start of its annual spring offensive on Wednesday, dismissing an offer for peace talks by President Ashraf Ghani and saying it will focus on U.S. forces in Afghanistan.

The announcement of the Al Khandaq campaign, named after the so-called Battle of the Trench, fought by the Prophet Mohammad to defend the city of Medina in the early days of Islam, marks the symbolic start of the fighting season.

But heavy fighting has been going on in different parts of Afghanistan. Hundreds of people have been killed and wounded in a series of high profile attacks in Kabul since the beginning of the year, despite Ghani’s offer in February for peace talks “without preconditions”.

A Taliban statement on Wednesday dismissed the peace overtures as a “conspiracy”.

“Their main effort is to deviate public opinion from the illegitimate foreign occupation of the country, as the Americans have no serious or sincere intentions of bringing the war to an end,” the Taliban said.

Acting U.S. Secretary of State John Sullivan said the Taliban announcement underscored the group’s “responsibility for the insecurity that destroys the lives of thousands of Afghans each year.”

“There is no need for a new ‘fighting season’,” Sullivan said. “Still, the Taliban announced another campaign of senseless violence targeting the democratically elected and internationally recognized Afghan government and their fellow Afghans.”

The militants, fighting to restore their version of strict Islamic law to Afghanistan, said their campaign was a response to a more aggressive U.S. military strategy adopted last year, which aims to force the militants into peace talks.

“Its primary target will be the American invaders and their intelligence agents. Their internal supporters will be dealt with as a secondary target,” the Taliban said.

Thousands more U.S. troops have been sent to Afghanistan to help train the army, and commanders have been given greater authority to carry out air strikes against the militants in a major reversal of the previous policy of phased withdrawal of American forces.

As in previous years, the Taliban pledged to protect the lives of civilians, and made no mention of incidents such as an attack it claimed in Kabul in January in which an explosives-packed ambulance blew up, killing about 100 people.

However, the announcement underlines the risk to parliamentary and district council elections scheduled for October as efforts to register voters in remote areas of Afghanistan get underway.

On Sunday, some 60 people were killed in a suicide bombing at a voter registration center in Kabul in an attack claimed by Islamic State. There have been a number of other attacks outside the capital.

Estimates of Taliban territorial control vary but the Pentagon estimates that 56 percent of the country is under government control, while a BBC survey this year estimated the insurgents were active in 70 percent of Afghanistan.