ISLAMABAD (REUTERS) - Pakistan's army on Wednesday said more than 100 soldiers had been killed by Taleban militants in the last five months, in a rare admission of mass casualties since the start of government efforts to engage the insurgents in peace talks.

The administration of Prime Minister Nawaz Sharif, who came to power last year promising to find a negotiated peace with the Taleban, has been trying to engage the militants in talks but these efforts have faltered in past weeks.

The lack of progress on peace talks has given rise to speculation that the government, backed by the powerful army, is preparing for a ground and air offensive against Taleban strongholds in the tribal belt on the Afghan border.

Requesting anonymity, security officials told Reuters that 308 civilians, 114 military personnel and 38 police officers had been killed since Sept. 2013, around a time when Sharif convened an all-party conference that unanimously opted for peace talks.

The Pakistani army almost never releases numbers of its losses against Taleban insurgents, a group operating seperately from its Afghan namesake, and striving to overthrow the Islamabad government and install an Islamic state.

Talks between representatives of the Taleban and government officials broke down earlier this week after a series of attacks on security forces claimed by fringe Taleban groups.

The army publicly supports Sharif's call for talks but in private senior officers have spoke strongly against it, giving rise to talk that the military are waiting for an excuse to mount a major operation against militant strongholds.

"An all-out operation is highly unlikely but a targeted operation is unavoidable," a senior military official told Reuters, requesting anonymity.

"There is a very strong realisation among all stakeholders that the Taleban will just never give up the use of force and so they will have to be dealt with the use of force. We've already wasted a lot of time over this charade of peace talks." Another intelligence official, speaking on condition of anonymity because he was not authorised to discuss military operational matters with journalists, said a military operation was in the works.

"It will not be carpet bombing," he said. "But the army is ready for a full onslaught to flush them out."