The strikes come in the final days of a year that has seen an unprecedented number of such drone-fired attacks as part of a ramped-up U.S. campaign to take out al-Qaida and Taliban fighters seeking sanctuary outside Afghanistan.
Around 115 missile strikes have been launched this year - more than doubling last year's total. Nearly all have landed in North Waziristan, a region that hosts several militant groups battling U.S. and NATO troops in Afghanistan, including the feared Haqqani network.
The first strike Tuesday hit a house in the Ghulam Khan area of North Waziristan, killing six, the Pakistani intelligence officials said. They spoke on condition of anonymity because they were not authorized to speak to the media.
The officials did not know the identities of those killed but said they were militants.
About three hours later, as people went to the site to pick up the bodies, more missiles hit the same spot. The intelligence officials said civilians may have been among those killed in the second strike.
Later in the day, nine militants were killed when six missiles hit their vehicles as they traveled in the Ghulam Khan area, the officials said. The dead were believed to be insurgents with the Haqqani network, which is considered one of the top threats to U.S. forces in Afghanistan.
Pakistan officially protests the strikes, saying they violate its sovereignty and anger tribesmen whose support it needs to fend off extremists. But Islamabad is widely believed to secretly support the strikes and provide intelligence for at least some of them.
U.S. officials rarely discuss the covert, CIA-run missile program. Privately, however, they say it is a crucial tool and has killed several top militant leaders. They also say the drone-fired strikes are very accurate and usually kill militants.
Information from Pakistan's tribal belt is very hard to verify independently. Access to the area is legally restricted, and ongoing conflict there makes it dangerous territory.
In Mohmand, another district in the tribal belt, a group of militants stormed a security checkpoint in the mountainous Ziarat area Tuesday, wounding two officers, said Zabit Khan, a government administrator. He said security forces returned fire and killed two of the attackers.
Also Tuesday, a low-intensity bomb exploded near a cafeteria at the Karachi University in the southern port city of Karachi, wounding at least two students, police said. Police official Naeem Khan said the explosive was in parcel and that officials were trying to determine who planted it.
Associated Press writers Ashraf Khan in Karachi and Habib Khan in Khar, Pakistan, contributed to this report.