"Although no final assessment has been made, it is believed insurgents suffered heavy casualties during several hours of fighting," NATO said in a statement.
U.S. officials say militant attacks in Afghanistan are becoming more complex, intense and better coordinated than a year ago. Monthly death tolls of U.S. and NATO troops in Afghanistan surpassed U.S. military deaths in Iraq in May and June. And last Monday, a suicide bomber attacked the Indian Embassy in Kabul, killing 58 people in the deadliest attack in the Afghan capital since 2001.
U.S. officials are considering drawing down additional forces from Iraq in coming months, in part because of the need for additional U.S. troops in Afghanistan. U.S. officials have said they need at least three more brigades in Afghanistan - or more than 10,000 troops.
NATO confirmed nine of its soldiers had been killed and 15 wounded. A Western official said the nine dead were Americans, speaking on condition of anonymity because he was not authorized to release the troops' nationalities. Four Afghan soldiers also were wounded, NATO said.
Lt. Col. Rumi Nielson-Green, the top U.S. military spokeswoman in Afghanistan, said she could not comment because the fighting was ongoing.
The attack was the deadliest for U.S. troops in Afghanistan since June 2005, when 16 American troops were killed - also in Kunar province - when their helicopter was shot down by a rocket-propelled grenade. Those troops were on their way to rescue a four-man team of Navy SEALs caught in a militant ambush. Three SEALs were killed, the fourth was rescued days later by a farmer.
The latest assault came at a time of rising violence in Afghanistan. Also on Sunday, a suicide bomber targeting a police patrol killed 24 people, including 19 civilians, while U.S. coalition and Afghan soldiers killed 40 militants elsewhere in the south.
More than 2,300 people - mostly militants - have died in insurgency-related violence this year, according to an Associated Press tally of official figures. Attacks in eastern Afghanistan are up 40 percent this year compared with last year.
Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, warned during a visit to Kabul last week that there are more foreign fighters, including al-Qaida members, in Pakistan's tribal areas, militants who cross the border and launch attacks against U.S. and Afghan troops.
Mullen has said he hopes improved security in Iraq will allow troops to be shifted this year from Iraq to Afghanistan, where violence is rising.
Violence in Iraq is at its lowest level in four years and Iraqi forces are taking on more responsibility, trends that could allow Gen. David Petraeus, the top American commander in Iraq, to recommend to President Bush in September that he resume a troop withdrawal that is being put on hold this month so Petraeus has time to assess the overall situation. A top Bush aide, Ed Gillespie, said Sunday that withdrawing more troops from Iraq after that assessment always has "been a possibility."
Another cause for concern in Afghanistan is the high casualty tolls for civilians killed in violence. This month, an Afghan government commission found that U.S. aircraft killed 47 civilians during a bombing run in Nangarhar province, while a separate incident in Nuristan province is alleged by an Afghan officials to have killed 22 civilians.
The tolls have prompted the International Committee of the Red Cross this week to ask all sides to show restraint and avoid civilian casualties. But violence continued around the country on Sunday.
A suicide bomber on a motorcycle blew himself up next to a police patrol in the southern province of Uruzgan, killing 24 people. The bomb attack on a police patrol at a busy intersection of the Deh Rawood district killed five police officers and 19 civilians, wounding more than 30 others, said Juma Gul Himat, Uruzgan's police chief. Most of those killed and wounded were shopkeepers and young boys selling goods in the street, he said.
Elsewhere, Taliban militants executed two women in central Afghanistan late Saturday after accusing them of working as prostitutes on a U.S. base.
The women, dressed in blue burqas, were shot and killed just outside Ghazni city in central Afghanistan, said Sayed Ismal, a spokesman for Ghazni's governor. He called the two "innocent local people."
Taliban fighters told Associated Press Television News the two women were executed for allegedly running a prostitution ring catering to U.S. soldiers and other foreign contractors at a U.S. base in Ghazni city.
1st Lt. Nathan Perry, a U.S. military spokesman, said he had not heard allegations "anything close to that nature."
Meanwhile, at least 40 militants were killed following an attack on Afghan and U.S.-led coalition forces in Helmand province, the coalition said in a statement. The militants attacked the combined forces near Sangin on Saturday from "multiple concealed and fortified positions," the coalition said. Thirty "enemy boats" and several small bridges have been destroyed on the Helmand River during two days of fighting, it said.
Also Sunday, a soldier with NATO's International Security Assistance Force died in a roadside blast in Helmand province, a statement said. The soldier's nationality was not released and it wasn't clear if the death was connected to the two-day battle.
In the north, a soldier serving with ISAF died of wounds caused by an explosion Saturday, the military alliance said in a statement. The statement did not give any further details of the explosion. The soldier's nationality was not disclosed.
There are nearly 53,000 troops from 40 nations serving in the ISAF in Afghanistan.
