BAGHDAD, Dec 5: At least 21 people, including 17 Iraqis working for the US military, were killed on Sunday as a senior UN official warned that elections could not be held in the current climate of violence.

With more than 90 people killed in the last three days in a spike of unrest despite the end of US-led assaults on rebel cities south and west of Baghdad, Sunni Muslims also stepped up calls to delay January's landmark polls.

Lakhdar Brahimi, a special adviser to UN Secretary General Kofi Annan and until recently UN envoy in Iraq, said the Jan 30 vote could only take place "if first and foremost security improves".

The United States has been forced to increase by early January the number of troops to about 150,000 from 138,000, the highest number since it declared an end to major combat, desperate to ensure the election process runs smoothly.

Early Sunday, 17 Iraqis working for the US military in Saddam Hussein's hometown of Tikrit were killed when men in two pick-ups raked their minibuses with a hail of bullets.

As weekend violence claimed the lives of four US soldiers, three Iraqi national guardsmen, including a regional commander, were killed in a car bombing near the oil refinery town of Baiji, just north of Tikrit.

One day earlier, 17 Kurdish militiamen, Peshmergas, were killed in the main northern city of Mosul when their convoy was rammed by a suicide bomber in a car close to the headquarters of the Patriotic Union of Kurdistan party.

Despite stressing his comments were meant in a personal capacity, Brahimi's remarks on the difficulty of holding polls have highlighted further concerns about whether the Iraqi elections can go ahead as planned.

The highly-respected Brahimi told the Dutch newspaper NRC Handelsblad in an interview that if the elections were to take place in Iraq's secure areas it would exclude the Sunni minority living in more tense regions. -AFP