BAGHDAD, Iraq (AP) - Insurgents exposed the vulnerability of Iraq's security forces again yesterday, killing at least 18 National Guardsmen, five police officers and several civilians in separate attacks with elections just weeks away.

Prominent Shiite leaders called for unity with Sunni Arabs wanting to delay the vote but insisted it be held despite the violence.

Also, the US military sent new forces to counter the threat in Mosul, centre of a worrying rise in car bombings and raids in recent weeks.

Secretary of State Colin Powell later repeated past warnings of more violence ahead of the January 30 elections for a national assembly, and the guerrillas have made good on those fears with tragic ease. Iraq's poorly-equipped security forces usually have far less training than American troops, and attacks on them usually result in far more casualties.

The worst attack yesterday occurred north of Baghdad when a suicide bomber detonated his explosives-laden vehicle next to a bus carrying Iraqi National Guard troops. It was the deadliest assault on Iraqi security forces since October, when insurgents gunned down about 50 new National Guardsmen at a fake checkpoint.

US military spokesman Maj Neal E O'Brien said 18 National Guardsmen and their driver were killed and six wounded. The Al-Jazeera satellite channel, however, said 25 troops were killed.

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