(CNN) -- NATO helicopters opened fire on a Pakistani checkpoint early Saturday, killing 26 soldiers, a provincial governor said.
At least 14 soldiers were wounded in the attack in the Mohmand Agency area, said Syed Masood Kausar, governor of northwestern Khyber Pakhtunkhwa province.
Mohmand Agency is one of seven districts of the volatile region bordering Afghanistan.
"It's a huge incident; the reaction is obvious," he said at a news conference in Islamabad.
The death toll could rise as many of the injuries are critical, military officials said. The officials did not want to be named because they are not allowed to talk to the media.
Pakistani Prime Minister Yousuf Raza Gilani has called an emergency meeting of military services chiefs to formulate his country's response, his spokesman said. The Defense Committee of the Cabinet will meet later Saturday.
In a statement, Gilani said he "strongly condemned the NATO/ISAF attack on the Pakistani" checkpoint.
The matter is being taken up by the Foreign Ministry "in the strongest possible terms" with NATO and the United States, the statement from his office said.
NATO's commander in Afghanistan said he is committed to a thorough investigation.
"This incident has my highest personal attention and my commitment to thoroughly investigate it to determine the facts," Marine Gen. John R. Allen said Saturday.
He also offered his "sincere and personal heartfelt condolences" to the families of any Pakistan Security Forces members killed or injured.
U.S. Ambassador to Pakistan, Cameron Munter, said: "I regret the loss of life of any Pakistani servicemen, and pledge that the United States will work closely with Pakistan to investigate this incident."
The incident on Pakistani soil is likely to damage already strained relations between Pakistan and the United States.
Pakistan closed both NATO's supply routes into Afghanistan Saturday in response to the attack, Pakistani military and intelligence officials said.
NATO trucks use the routes, in Khyber Agency and Balochistan, to supply U.S. and international forces fighting in Afghanistan.
About 50 containers and trucks carrying supplies for NATO were stopped at the town of Jamrud in Khyber Agency on Saturday morning, said Jamil Khan, a senior government official in Khyber Agency, bordering Afghanistan.
They were ordered to turn back toward Peshawar, the provincial capital of Khyber Pakhtunkhwa province, he said.
A second route from Pakistan into Afghanistan, the Chaman border crossing in Balochistan province, was open to NATO supply trucks earlier Saturday but was then shut in the afternoon, the military and intelligence officials said.
Roughly 40% of nonlethal NATO supplies and fuel go through Pakistan, with hundreds of supply trucks using the two routes into Afghanistan.
About 130,000 troops are deployed in Afghanistan with the NATO-led International Security Assistance Force, 90,000 of them American, according to NATO figures.
Pakistani politicians responded angrily to the incident in Mohmand.
Ahmed Khan Bahadur, a provincial lawmaker from the Awami National Party, the ruling party of Khyber Pakhtunkhwa province, told CNN that people had had enough.
"This is the time to be united as a nation and to punch NATO with a fist," he said. "NATO could never dare if we were united."
Former international cricketer turned politician Imran Khan, who heads the Pakistan Tehreek-e-Insaf party, condemned the incident and said it was time for Pakistan to pull out of the U.S.-led "war on terror."
If confirmed, the incident could be the deadliest for Pakistani soldiers involving NATO since a U.S. airstrike in June 2008, which Pakistan said killed 11 of its forces who were cooperating with the U.S.-led war in Afghanistan.
That airstrike, also in Mohmand Agency, prompted the government in Islamabad to summon the U.S. ambassador and lodge an official protest.
NATO's Allen met with the Pakistani Chief of Army Staff Gen. Ashfaq Parvez Kayani on Thursday, the Pakistani military said.
"The visiting dignitary remained with him for some time and discussed measures concerning coordination, communication and procedures between Pakistan army, ISAF and Afghan army, aimed at enhancing border control on both sides," a Pakistani military statement said.
Meanwhile, a commander of Afghanistan's eastern border police said an operation in the area bordering Mohmand Agency on Friday night killed 10 insurgents.
"Last night, there was an operation there inside Afghanistan," he said. "Pakistani and Afghan Taliban have got a broad presence there as there are forests and difficult terrain. That's why there was an operation."
The military activity was in Afghanistan's Kunar province, he said, adding he was unaware of any NATO attacks on the other side of the border.
CNN's Reza Sayah and Nasir Habib in Islamabad contributed to this report.