Suspected Al Qaeda militants shot dead a Yemeni intelligence officer in the southern city of Mukalla on Monday, a security official told Reuters.

Al Qaeda in the Arabian Peninsula (AQAP) has carried out a series of hit-and-run attacks since the Yemeni army drove it from its southern strongholds in Abyan and Shabwa provinces in April.

Gunmen on a motorbike killed Colonel Ahmed Radman, a member of Yemen's political security division, the source said. At least 25 other senior officers have been killed in Yemen this year.

Another colonel was shot dead in Sanaa on Saturday and an officer and a local official were killed in two Southern provinces on Sunday, officials told Reuters.

The West is concerned AQAP could use Yemen, which borders major oil producer Saudi Arabia, as a base for international attacks.

In another major develoment, security sources said a planned ceasefire between the Yemeni government and Shi'ite rebels had failed to take effect, hours after both sides published the terms of a truce to end fighting in and around the capital Sanaa on Monday.

Security sources said clashes continued in the city of Amran in Yemen's north.

The Houthi insurgents - named after the tribe of their leader - as well as the defence ministry's newspaper, 26 September, posted details of the ceasefire on their websites late on Sunday.

But a local official said it had yet to go into effect.

"What was published in the media as a ceasefire is in actual fact just a vision of how to end the war," Ahmed al-Bekry, deputy governor of Amran said on his facebook page, adding that neither the military nor the Shi'ite Houthi rebels had signed the deal.

Since the breakdown last week of a previous truce reached at the beginning of the month, government forces and allied tribesmen have struggled to halt the militants' advance on villages just 40 km northwest of the capital.

Policemen clashed with armed followers of the Houthi movement in a standoff outside the group's political bureau in Sanaa on Saturday.

Yemen has been in turmoil since 2011 when mass protests forced veteran president Ali Abdullah Saleh to step down.

At least 200 people have been killed this year in battles pitting the insurgents against the government and tribal allies.

Officials say the Houthis, who have fought short but devastating wars with government forces since 2004, are getting weapons from Iran. The rebels deny this, saying they seek autonomy and less US interference in Yemen's affairs.

The fighting in northern Yemen, which has taken on a sectarian tone, has further unsettled a country struggling to overcome many problems, including a secessionist movement in its restive South and the nationwide spread of Al-Qaeda militants.-Reuters