During the course of the attack, heavy gunfire and multiple explosions could be heard coming from terminals, as Taliban fighters and security forces battled for control of the airport’s main building.
Ten attackers carried out the operation, according to the chief minister of Sindh province, Qaim Ali Shah.
"They were well trained. Their plan was very well thought out," he told reporters. He said the attackers intended to destroy aircraft and the airport’s main building but were unsuccessful in their attempt.
Officials said weapons involved in the raid included submachine guns, rocket-propelled grenades, grenades and explosives.
At least some of the gunmen wore the uniforms of the Airport Security Force, which protects Pakistan's airports, and all the men were strapped with explosives, said an official with one of the country's intelligence agencies who briefed journalists near the airport. He declined to give his name.
Pakistan's paramilitary force said that the attackers were ethnic Uzbeks, reported Reuters. Pakistani officials often blame foreign rebels holed up in lawless areas on the border with Afghanistan for staging attacks alongside the Pakistani Taliban.
"Three militants blew themselves up, and seven were killed by security forces," Rizwan Akhtar, regional head of the paramilitary Rangers, said in televised remarks. "The militants appear to be Uzbek."
Authorities diverted incoming flights and suspended all flight operations. A spokesman for the Civil Aviation Authority said the airport would be closed until at least Monday night.
Sarmad Hussain, an official with the state-run Pakistan International Airlines, was working at the airport when the attacks occurred.
"I was working at my office when I heard big blasts — several blasts — and then there were heavy gunshots," he told The Associated Press after escaping the building.
Hussain said he and a colleague jumped out one of a window to get away, and his colleague broke his leg.
By dawn on Monday, the military reported that the airport had been secured, but heavy smoke continued to rise from it, and numerous reports from international media indicated the fighting may have resumed.
Karachi is Pakistan's largest city and has been the site of other attacks. It is the country's economic heart, and any activity targeting the airport will likely strike a blow to foreign investment in the country.
Suicide bombers in southwestern Pakistan on Sunday killed 23 Shia pilgrims returning from Iran. Although unrelated, the incident underscored the fragility of Pakistan's security.
Pakistan's government has been trying to negotiate a peace deal with armed groups mostly based in the northwest that have been waging war against the government. But the talks have had little success, raising fears that the groups will increase attacks across the country.
Security officials in Karachi feared that if talks broke down, Karachi would be a likely spot for armed groups to strike back because the Pakistani Taliban and their allies have increasingly gained a foothold in the city in recent years.
In a statement posted on Twitter shortly after the attack, the Taliban promised more violence.
"We have yet to take revenge for the deaths of hundreds of innocent tribal women and children in Pakistani air strikes. It's just the beginning. We have taken revenge for one. We have to take revenge for hundreds."
Al Jazeera and wire services
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