Information Minister Sheikh Rashid Ahmed said on Thursday that the incident took place on Tuesday
"They are Arabs but their nationality is not yet known," he said.
Also on Thursday, Aljazeera television said two suspected al-Qaida members had been arrested in Quetta on Monday.
The television station said it had also received a statement that identified one of the two as Mustafa al-Sit Mariam Nasar, a Syrian wanted for questioning in connection with the Madrid bombings in March 2004.
The Pakistani minister said that could not be confirmed and authorities were still trying to determine if the arrested man was indeed al-Sit Mariam Nasar,who has a bounty of $5 million.
"They are Arabs; but their nationality is not yet known"
Information Minister Sheikh Rashid Ahmed
The US Justice Department's Rewards for Justice website,too, confirms there is a $5 million reward for the capture of Nasar, 47, adding that he was believed to be in Afghanistan or Pakistan.
Role in Madrid attacks
Aljazeera television said that the two suspects were arrested last Monday in Balochistan's capital of Quetta in south-eastern Pakistan.
The channel's office in Pakistan also received a statement, purportedly from al-Qaida, saying that Mustafa al-Sit Mariyam also known as Abu Musab al-Suri, was arrested without referring to the place of arrest.
Al-Sit Mariyam is a Syrian with Spanish nationality and was among the Muslim Brotherhood members in Syria who fled to Afghanistan.
Spanish authorities said he was behind Madrid's attacks.
According to the US website, Nasar is an al-Qaeda member and former trainer at two terror camps in Afghanistan who pledged allegiance to Osama bin Laden after the 11 September, 2001 attacks on the United States.
The 11 March, 2004 Madrid train
bombings killed 191 people
It says Nasar worked with another key member to train recruits in the use of poisons and chemicals.
The website says, "recent unconfirmed press reports suggest that he may have had a role in the 11 March, 2004 Madrid Bombings". The attacks on commuter trains killed 191 people.
Media and security journal reports have also quoted intelligence officials in Britain as saying that Nasar is wanted in connection with the 7 July London bombings, which left 56 people dead.