"It can carry all types of warheads," Major-General Shaukat Sultan told Reuters, adding that it has a range of up to 1500 km (930 miles).
The test of the Ghauri (Hatf V) missile came a week after a new government took office in India and pledged to continue a peace process with Pakistan but postponed talks aimed at easing nuclear tensions, saying it needed time to settle in.
There was no immediate reaction from India but an Indian defence ministry official told Reuters the test had been expected.
Pakistan says its weapons programme is a response to that of India, with which it has fought three wars since both countries won independence from Britain in 1947.
Analysts say the test was meant to tell the Indian government that Pakistan would not lower its guard despite peace moves.
"It's meant to meet technical requirements, ease domestic pressure on peace with India and also convey to the new Indian government that we are going to remain militarily strong," former military general Kamal Matinuddin said.
"It's a combination of all."
A Pakistan military statement said Prime Minister Zafarullah Khan Jamali also watched the test-firing, but did not say where the test was done.
"The Ghauri missile....is second only to Hatf VI Shaheen II in terms of range," it said.
Ghauri and Shaheen are different versions of a Pakistani missile series named Hatf in reference to an ancient Islamic weapon.
Pakistan test-fired the Shaheen II, with a range of 2,000 km (1,250 miles), in March. It said it was capable of carrying nuclear warheads to every corner of India.
New Delhi has itself been developing a range of missiles capable of delivering nuclear warheads, and plans to test sometime this year the longest-range version yet of Agni, its nuclear-capable ballistic missile.
Pakistan had informed its neighbours about the test beforehand, the military statement said.
DAY AFTER SIXTH TEST ANNIVERSARY
Pakistan first test-fired the Ghauri missile in April 1998 ahead of tit-for-tat nuclear tests responding to tests by India. The Ghauri missiles were formally inducted into the military in January 2003.
The latest Ghauri test came a day after the sixth anniversary of Pakistan's nuclear tests.
The military statement said the test aimed to confirm improvements in the Pakistani missile system.
The Ghauri missile was developed by Khan Research Laboratories, Pakistan's main uranium-enrichment facility, which was named for Abdul Qadeer Khan, once revered as the father of the country's atom bomb.
But Khan was sacked this year from his job as a special government adviser after he admitted to exporting nuclear secrets to Iran, Libya and North Korea.
Some experts say the Ghauri missile was developed with North Korean help in return for nuclear know-how.
(Additional reporting by Sanjeev Miglani in NEW DELHI)