(Updates with comment from governor, U.S. military)

By Sayed Salahuddin

KABUL, April 17 (Reuters) - About 100 Taliban raided Afghan police posts and a district government headquarters northeast of Kabul on Tuesday, in the heaviest fighting in an area so close to the capital since 2001.

Earlier on Tuesday, a bomb blew up a U.N. vehicle in the city of Kandahar killing four Nepali contractors and an Afghan driver. The Taliban claimed responsibility. Separately, four children were killed in a blast at a school in the city of Herat.

The Taliban launched coordinated attacks in the rugged Tagab district of Kapisa province, 70 km (42 miles) from Kabul. Heavy fighting went on for hours and the government requested and received U.S. military support, the provincial governor said.

"The fighting is heavy and has been going on since the morning. U.S. air support is also involved," said the governor, Abdul Sattar Murad. Several Taliban had been killed, while one policeman was killed and four wounded, he said.

A U.S. military spokesman said there had been "activity" in the area but he had no details. Another foreign official said heavy clashes had broken out along a 10-km (6-mile) front.

Violence in Afghanistan surged last year to its worst level since the Taliban were ousted in 2001.

Fighting eased over the winter, as it traditionally does in Afghanistan, but attacks have been picking up over recent weeks.

The Taliban have been vowing to launch a spring offensive backed by thousands of suicide bombers.

The fighting in Kapisa, in a district north of the town of Sarobi, subsided after darkness fell and Murad said the Taliban had not managed to capture any government positions.

NATO and U.S.-led forces have been mounting sweeps in the south to thwart the threatened offensive, but apart from an occasional clash, Kapisa has been peaceful.

TALIBAN THREAT

The Taliban claimed responsibility for the remote-controlled bomb set off as a U.N. convoy passed in the southern city of Kandahar that killed the four Nepalis and the Afghan driver.

The Nepalis were contractors working with the U.N. Office for Project Services. It was not clear if the Afghan driver was a contractor or had been working for the United Nations, a U.N. spokesman said.

"Intentional attacks on civilians are a clear violation of international humanitarian law and the U.N. will be pursuing full accountability for those who are behind this," the United Nations said in a statement.

A Taliban commander, Mullah Hayatullah Khan, claimed responsibility, saying people helping foreign forces were targets.

"We'll target all individuals or organisations that are either cooperating with coalition forces or working under their supervision," Khan said by satellite telephone from a undisclosed location.

The Taliban have killed dozens of aid workers since 2001 but Tuesday's attack was the worst on people working for the United Nations in Afghanistan since then.

A spokesman for President Hamid Karzai said efforts were under way to secure the safe release of two French aid workers, abducted by the Taliban along with three Afghans in the southwestern province of Nimroz on April 5. He did not elaborate.

Police in the western city of Herat blamed the Taliban for a blast in a school compound that killed four students and wounded several. (Additional reporting by Saeed Ali Achakzai)

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