More NATO (search) troops are urgently needed to secure Afghanistan ahead of September elections, the top U.N. envoy in the country said Monday after militants attacked a voter registration office near the capital.

Assailants also killed an Afghan soldier, and injured a second, who were deployed to protect election workers in southern Kandahar province.

Jean Arnault said that a wave of violence — including recent deadly attacks on foreigners and relief workers — showed increased "volatility" ahead of the polls, and said "the time is now" for NATO to boost its presence.

"The events of the past three weeks have demonstrated that security is not improving. If anything it has become more volatile," he told a news conference in Kabul.

Early Monday, attackers fired three rocket-propelled grenades at the registration office in the main town of Logar province, about 35 miles south of Kabul, damaging the building but hurting no one.

Gen. Atiq Ullah Ludin, the provincial commander, blamed rebels of the former Taliban (search) regime and renegade warlord Gulbuddin Hekmatyar (search), who have vowed to sabotage the vote — seen as key to rehabilitating a country shattered by war.

Arnault expressed concern over an attack so close to the security blanket provided by the existing 6,000 NATO-led peacekeepers in the capital.

"We are now facing attacks directed at us with fairly heavy weapons," he said.

NATO has promised for months to deploy more forces across the north of the country, but nations have been slow to commit troops. NATO leaders are due to discuss the issue at a June 28-29 summit in Turkey.

Security concerns have deepened this month, with the slaying of 11 Chinese road workers and five aid workers, three of them Europeans, in northern provinces, once regarded as relatively stable.

Meanwhile, electoral teams have come under repeated attack in the lawless south and east, where the Taliban and Al Qaeda rebels are strongest, leaving two British U.N. contractors dead in May and several Afghans wounded.

On Monday, assailants on two motorbikes sprayed a vehicle carrying two Afghan soldiers, killing one and wounding the second, as they traveled to an election office in Arghandab district, about 20 miles north of Kandahar city.

"The soldiers were our security guards," said Shoaib, a regional election coordinator, who like many Afghans goes by one name. "But this act won't stop the (voter) registration program."

The U.S. military, which has 20,000 troops hunting insurgents in those regions, gave a more upbeat assessment of the security situation.

Spokesman Lt. Col. Tucker Mansager said Monday that allegations of deteriorating security "fly in the face of the successes we are seeing in voter registration."

So far, just over 4 million have signed up — less than half the estimated number of eligible voters.

Arnault said there was public willingness and determination to take part in the election and he hoped that between 8 million or 10 million would sign up by the end of July.

He also urged warlords to cooperate with lagging, U.N.-backed efforts to disarm militiamen, with only 9,000 laying down arms so far — against an end-of-June target of 40,000.