MINGORA, Pakistan (Reuters) - Pro-Taliban militants killed three army intelligence officers in an ambush in the northwestern Swat valley, officials said on Tuesday, delivering a serious blow to a peace accord signed in May to end violence.

The peace agreement with militants led by radical cleric Fazlullah brought a brief lull after months of violence in the valley, but militants have stepped up activity in recent weeks, accusing the government of failing to honor the accord.

The three officials, believed to be from an army intelligence agency, were returning to Mingora, the main town of Swat, when gunmen sprayed their vehicle with bullets late on Monday, killing them on the spot.

A spokesman for Fazlullah’s group accepted responsibility.

“We have killed them because people from the security agencies arrest our people and brutally torture them,” the spokesman, Muslim Khan, told Reuters.

The government officials confirmed the death of the three men, all of them from the army, but refused to say which security agency they belonged.

After the attack, security forces fired mortar bombs on the militants hideouts and also arrested six suspects in a search operation early on Tuesday.

Before the Islamist militants launched a campaign of violence last year, the alpine Swat valley in North West Frontier Province had been a top tourist destination.

Fazlullah’s faction was joined by militants from neighboring tribal areas bordering Afghanistan in a fight to enforce a Taliban-style Islamic rule in the valley.

The army began battling the militants in November, but the provincial government signed a peace pact with insurgents in May. Hopes that stability would return have been dashed by the recent surge in violence.

Also on Tuesday, militants torched a health office and exchanged fire with police in Matta, one of the militants’ stronghold.

The militant spokesman said their fighters would remain active unless troops were withdrawn from the valley.

(Reporting by Junaid Ali; Writing by Augustine Anthony; Editing by Simon Cameron-Moore and Alex Richardson)