KARACHI, Pakistan (AP) — A suicide bomber smashed a truck packed with explosives into housing for a paramilitary force protecting Pakistan’s largest city Thursday, killing three officers in the explosion that sent a large plume of smoke into the sky, officials said.

The blast underlined deteriorating security in Karachi, the sprawling port city of 18 million people that is considered the economic heart of Pakistan. Violence has escalated in recent years in the city as armed groups fight for control of land and resources, and militant groups like the Taliban have used the chaos to consolidate their foothold.

Three security personnel were killed and 20 were wounded in the attack, said a spokesman for the Rangers, speaking on condition of anonymity in line with regulations.

He said the blast would not deter the Rangers from pursuing operations against militants in the city.

The Rangers are a paramilitary force that is tasked with helping Karachi police maintain security in the city.

A truck driven by a suspected suicide bomber smashed into the gates of a residential apartment block where members of the Rangers security forces live, said Javed Odho, deputy inspector general of the Karachi police.

Witnesses reported seeing a large plume of smoke in the sky. Pakistani television images of the blast sight showed what appeared to be an apartment block with a gaping hole in the middle where the bomb went off and part of the two-story building was razed.

Rangers created a perimeter around the building to hold off journalists and bystanders.

One of the Rangers, Muhammed Farooq, said he was preparing for work when he looked out the window and saw a vehicle smash through the main gate and into the building.

"Then there was a really big bang and I lost my balance and I saw a lot of smoke and then I lost consciousness," he said, speaking from the hospital.

There was no immediate claim of responsibility. Taliban militants are known to operate inside the city and have targeted security officials and buildings in the past.

Half a dozen Taliban militants attacked a major naval base in Karachi in May 2011, killing at least 10 people and destroying two U.S.-supplied surveillance aircraft. In September 2011, a Taliban suicide bomber detonated a vehicle packed with explosives outside the home of a senior police officer tasked with cracking down on militants in Karachi. At least eight people died, although the officer survived.

Karachi is in southern Pakistan and is the capital of Sindh province. The sprawling city along the Arabian Sea is the country’s wealthiest but has also been beset by escalating violence.

Armed groups backed by political parties are believed to be behind much of the violence in the city such as targeted killings, kidnappings and extortion. The chaos has allowed militants such as the Taliban, who’ve long had a presence in the city, to strengthen their presence there.

The Pakistani Supreme Court last week held hearings examining the violence, which some worry threatens Karachi’s stability.