Updated

The United Nations (search) has encouraged some nonessential staff to leave Afghanistan (search) amid security concerns ahead of Sept. 18 elections, and the government warned aid workers Sunday that they are likely targets after a string of assaults on foreigners.

The insurgents, meanwhile, launched a fresh spate of guerrilla-style strikes this weekend, sparking fierce battles that killed a district police chief, seven officers, an election candidate and three others, officials said.

More than 1,100 people have been killed in the past six months, and U.S. military commanders believe the violence may worsen as rebels step up attacks with legislative elections just two weeks away, the next key step toward democracy after a quarter century of fighting.

U.N. spokesman Adrian Edwards told The Associated Press that some of the world body's agencies had urged some employees to take vacation during the elections because of fears of violence, though he noted that the official U.N. alert level had not changed.

"The United Nations continues to monitor the security situation," he said.

Emma Sutcliffe, a U.N. Development Program (search) communications associate, said "We have been encouraged to take R and R (rest and recreation), but it's not mandatory."

"For those who remain behind, there'll be minimal movement," she said.

Other U.N. agencies though, including the World Food Program (search) and the U.N. High Commissioner for Refugees (search), said they had not increased their security precautions.

U.S. Embassy spokesman Lou Fintor said the security status at the heavily guarded mission had not changed but stressed the threat situation was constantly being monitored.

News that some U.N. staff were being encouraged to leave came after Interior Ministry spokesman Latfullah Mashal warned of more attacks.

"They (militants) focus on soft targets, attacking candidates, burning schools, aid workers," he told AP. "But the security workers have also taken necessary measures to provide needed security."

He said he was optimistic that the elections would be successful even though "al-Qaida and the Taliban will try their best to disrupt peace and stability."

Some of the latest violence has been against foreigners, including the killing of a kidnapped British engineer and the separate slaying of two Japanese teachers, spiking fears among the roughly 3,000-strong expatriate community in Kabul.

The body of Briton David Addison was found Saturday when American commandos stormed a suspected Taliban hide out in western mountains.

Addison was working for a foreign company building a road from Kandahar to the western city of Herat when he was abducted Wednesday.

Mashal said the kidnappers killed him immediately after taking him hostage.

In the latest violence, suspected Taliban rebels ambushed a district police chief in Helmand province Sunday while he was driving, killing him, three of his officers and his son, said local Gov. Amanullah Khan.

Two militants were killed when the police shot back at the attackers, he said.

Another three policemen were killed Saturday when rebels attacked a convoy of trucks they were guarding that was carrying supplies to a U.S. base in southern Zabul province, said local government chief Rozi Khan on Sunday.

Rebels also attacked a police checkpoint on the main road from the southern city of Kandahar to the capital, Kabul, on Saturday, triggering a two-hour gunbattle that left one officer dead and two wounded, local police chief Ghulan Nabi said.

Some insurgents were believed killed in the fighting as well, but it was unclear how many because the rebels took the bodies with them when they fled, he said.

Also in the south, in Helmand province, a bomb blew up Sunday outside the home of election candidate Hadidullah Khan, killing him, local police chief Din Jan Khan said, blaming the Taliban for the attack.

At least five candidates and four election workers have been killed in recent weeks.

A sixth candidate was feared dead after he was kidnapped Friday along with a district governor and three of their friends in Kandahar province. The Taliban claimed that they had killed all five, but Kandahar Gov. Asadullah Khalid said this could not be confirmed.

In a separate development, security forces discovered two big bombs Sunday hidden under a bridge in Kandahar city, Khalid said. It was not clear when the bombs were meant to explode.