Islamabad, Pakistan (CNN) -- Gunmen opened fire on a convoy of trucks in northwestern Pakistan that were carrying provisions for NATO forces in Afghanistan, killing two people.
It was the first attack on the supply routes since they were reopened earlier this month, a local official said.
The assault was carried out by more than 12 men riding motorbikes, said Ubaid Ullah, an official in Khyber Agency, the district where the convoy was attacked. Khyber Agency and six other districts are part of Pakistan's volatile tribal region that borders Afghanistan.
Two people were killed, and three others were wounded, Ullah said.
No one immediately claimed responsibility for the attacks.
But after Pakistan reopened routes earlier this month, the Pakistani Taliban threatened to attack NATO trucks, saying the supplies are used to target its members fighting against occupation in Afghanistan.
Transporters who resume supplies will be "considered a friend of the U.S." and will face the consequences, a spokesman for the militant group said then.
Taliban have attacked NATO supplies in the past, and killed drivers and other crew.
On July 5, trucks carrying supplies to NATO troops crossed from Pakistan into Afghanistan for the first time in seven months after Islamabad agreed to reopen routes, officials said.
Because Afghanistan is landlocked, many supplies for NATO-led troops fighting Islamic militants have to be trucked in from Pakistan.
On July 3, Islamabad decided to reopen the crucial supply routes shut down on November 27, a day after coalition forces mistakenly killed 24 Pakistani troops.
The incident plunged U.S.-Pakistan relations to an all-time low.
The Pakistani routes offer a shorter and more direct route than the one NATO has been using since November that went through Russia and other nations, avoiding Pakistan altogether.
It has cost the U.S. $100 million more a month to use the alternative northern routes.
The talks to reopen routes had been stuck on two key issues -- Pakistan's demand to charge more per container shipped across its border, and Pakistan's demand that the United States apologize for the friendly fire incident in November 2011.
Eventually, U.S. Secretary of State Hillary Clinton did.
"We are sorry for the losses suffered by the Pakistani military. We are committed to working closely with Pakistan and Afghanistan to prevent this from ever happening again," Clinton's statement said on July 3.
Until her apology, the U.S. government had only expressed regret over the incident, but had not issued a direct apology.
Pakistan has now agreed not to impose any transit fee with the reopened routes, Clinton said in a statement.
The Pakistani route costs about $250 per truck. Pakistan had been seeking $5,000 per truck as a condition of reopening the supply lines, which the United States refused to pay.
The U.S. military will now pay Pakistan $1.1 billion it owes as part of the deal struck to reopen the NATO supply lines, according to U.S. officials who had knowledge of the agreement's details but weren't authorized to speak publicly.
The money is part of a U.S. military program called "coalition support funds," which reimburses the Pakistani military for counterterrorism efforts. The U.S. halted paying the bills from Pakistan as tension rose between the two countries.
The Pentagon will consult with Congress about paying the bills prior to paying Pakistan in full, according to one of the U.S. officials.
Journalist Aamir Iqbal contributed to this report.