Most victims of the Kandahar blasts were children

Kofi Annan said in a report to the Security Council that continued violence could jeopardise national elections scheduled for June this year.

He was speaking after two explosions in the southern city of Kandahar killed at least 15 people, most of them children.

More than 400 people, including militants, have been killed in unrest in the past six months.

Critical challenges now face the process, and Afghanistan and the international community will need to take further steps UN Secretary General Kofi Annan

On Wednesday, it emerged that 12 ethnic minority Hazaras had died in another attack late on Tuesday in Afghanistan's troubled south.

The men were travelling in Helmand Province, which neighbours Kandahar Province, when unidentified gunmen attacked them, local officials said.

Mr Annan warned that registering voters for the elections could not be accomplished while many areas remained unsafe for UN teams.

The elections are a key requirement of the Bonn agreement on political reform, signed after the fall of the Taleban.

'Clear plan' needed

"The peace process in Afghanistan has reached a criticial juncture," Mr Annan said.

"Critical challenges now face the process, and Afghanistan and the international community will need to take further steps, expeditiously, if the process is to be successfully concluded."

Foreign workers and soldiers remain on their guard

The UN secretary general said the time had come for a second Bonn conference to mobilise international support for a country slowly and painfully emerging from 25 years of war.

"A clear plan, coupled with finances conditional on its implementation, will provide a strong signal that the resolve of the Afghan leadership and the international community remains firm," he added.

The BBC's Crispin Thorold in Kabul says peace-building in Afghanistan has long been a two-track process, with political changes continuing apace in the capital while insecurity is part of daily life in the provinces.

A third of the country is off-limits to the international community, particularly the south and south-east.

Fragile unity

In the latest violent incident, two bombs exploded near a military base and two schools in the southern city of Kandahar on Tuesday, killing 15 people and injuring nearly 60.

Witnesses said the second blast killed people who had gathered at the scene of the first.

Police in Kandahar said one man had been arrested at the scene of the explosions, but had said nothing under questioning.

On Sunday, delegates to a grand council meeting or Loya Jirga agreed Afghanistan's first post-Taleban constitution, the latest in a series of political reforms.

The new constitution envisages a powerful presidency for Afghanistan - as demanded by its current leader, Hamid Karzai - and a parliament with guaranteed representation for women.

The agreement came after three weeks of intense deliberations that, correspondents say, provided further evidence of Afghanistan's fragile ethnic relations.