Islamabad, Pakistan (CNN) -- The NATO attack that killed two dozen Pakistani troops on the border with Afghanistan last month appeared to be deliberate, two Pakistani lawmakers said Friday, ratcheting up the war of words with Washington over the controversial airstrikes.
"We were told there are indications that this was a deliberate attack because nothing else makes sense," said Sen. Tahir Mashhadi, who attended a briefing by Maj. Gen. Ashfaq Nadeem, a senior Pakistani military official, on Thursday night.
The NATO forces "knew very well these were Pakistani check posts," Mashhadi said, referring to the places where the troops were hit. "The check posts are clearly marked on all the maps. There is no explanation why they attacked and there's certainly no explanation why they attacked a second check post."
The NATO airstrikes on November 26 that killed 24 Pakistani soldiers have strained relations between Islamabad and Washington.
The United States has insisted that the attack was not deliberate and expressed regret over the deaths of the soldiers.
But in the aftermath of the attack, the Pakistani government has shut down NATO supply routes through Pakistan, boycotted a conference on the future of Afghanistan and also asked the United States to vacate an air base on its territory.
The attack prompted widespread outrage in Pakistan, and the military is conducting an investigation.
"Having listened to the briefing, this appears to be a deliberate attack," said Sen. Tariq Azeem, another lawmaker who attended the meeting with Nadeem. "We were shocked that such an incident could take place by our supposed ally. They had clear knowledge that these were Pakistani check posts,"
Both lawmakers said Nadeem could not say why U.S. and NATO forces would deliberately attack Pakistani check posts.
"That's the million dollar question," Mashhadi said. "I think it's important that we get an explanation from NATO. We are are on the same side. We are not enemies, but these are important questions that need to be answered if we want to continue to work together."
Several Pakistani newspapers reported Nadeem's comments Friday.
"NATO attack was planned," read a headline in The Nation, an English-language daily.
A spokesman for the Pakistani military declined to comment on Nadeem's briefing to lawmakers.
CNN's Reza Sayah and journalist Nasir Habib contributed to this report.