In Afghanistan, concerns are growing about security for the country's presidential election later this month amid an increase in violence. At least nine NATO troops have died in clashes with insurgents in Afghanistan over two days, and militants have stepped up attacks on election candidates and their staff. The top U.N. official in Afghanistan said any comprehensive effort to end the violence would have to include talks with top-level Taliban leaders.

The killing of more than 70 foreign soldiers in July made it the deadliest month for the U.S.-led coalition fighting the Taliban insurgents in Afghanistan. But there is no let up in the bloody trend in August as the country prepares to hold its second presidential election since the removal of the Taliban from power in 2001.

NATO officials say that armed militants in an eastern Afghan region attacked U.S troops on Sunday, soon after hitting their convoy with a roadside bomb. The clash left three American soldiers dead. Six other troops, including three Americans, were killed Saturday.

Authorities say Taliban extremists and their allies are behind increased attacks on election-related activities across the country.  

The latest attack took place on Saturday, when suspected Taliban militants ambushed a convoy carrying Afghan President Hamid Karzai's campaign workers. The attack killed one guard and wounded two others.

Last week, President Karzai offered peace talks to Taliban militants who are prepared to renounce violence.  Mr. Karzai's offer was echoed by U.S. and British officials, who called for the reintegration of Taliban fighters into Afghanistan's political system, if they renounce violence and extremism.

At a news conference in Kabul on Sunday, the U.N. Special Representative in Afghanistan, Kai Eide, said peace talks should include top Taliban leaders.

"If you want relevant results, you have to talk to those who are relevant," he said. "If you only have a partial reconciliation process, you will have partial results. If you do want a comprehensive peace process, it is not enough to talk to commanders on the ground. It is a political process, and I think you also have to approach the more political structures of the insurgency movement."

The U.N envoy did not name any insurgent leaders, such as fugitive chief of the Taliban insurgents, Mullah Omar. The militant leader has rejected previous offers of peace talks by President Karzai, saying no dialogue can take place while U.S-led coalition forces are present in Afghanistan. But Mr. Karzai has dismissed this demand, saying the presence of international troops is in the interest of his country.

While expressing concerns over alleged irregularities and Taliban attacks on those taking part in the election, U.N. Representative Eide, said the presidential election is most challenging for both the Afghans and the international community.  

"First of all, this is a country in a conflict. Secondly, it is a country with weak institutions, and it is a country with weak infrastructure and very high illiteracy rate," he said. "It is the most complex election, I have ever witnessed. This being said, it is my assessment that, what we have seen is by and large a dignified political campaign. We have never witnessed such a vibrant political debate in this country. And, we have never seen such involvement by the public as we have seen during these few weeks."

The stepped up militant violence also comes as U.S.-led forces are engaged in a major offensive in southern parts of the country aimed at improving security ahead of the August 20 election, with President Karzai seen as the front-runner from among as many as 40 candidates.