Four people were reportedly injured Thursday when a natural gas pipe exploded while UGI contractors were working to replace it, officials said.

The explosion happened shortly before 1 p.m. in the 200 block of North Fulton St., near West Turner St., in Allentown, fire officials said.

Reading-based UGI said crews were replacing a cast iron main in front of 213 when the blast occurred.

Something happened to cause either a spark or rupture in the old line, Mayor Ed Pawlowski told 69 News.

"I hear like boom and like the walls were moving a little bit," said Yassine Ellaouy.

It was a sight and sound some Allentown residents have grown to fear since a massive gas explosion killed five people last February.

Thursday's gas explosion was powerful enough to tear apart a porch, scattering bricks across the street and leaving a metal post dangling from a tree limb.

"So it was a flash and pressure explosion caused the damage that you saw up there, and then it went out," said Asst. Chief Lee Laubach, Allentown Fire Dept.

Four people -- two contractors and two residents -- were taken to local hospitals, Laubach said. The victims suffered minor injuries caused by flying debris.

The Red Cross is assisting the two families that have been displaced from the home's two apartments.

Nearby residents who were evacuated after the blast were allowed to return to their homes a short time later.

Thursday's explosion happened a little more than a month shy of the first anniversary of the Feb. 9, 2011 blast that sparked a massive fire, killed five people and destroyed several homes in Allentown.

That explosion raised more questions about the safety of the country's aging network of natural gas pipelines.

U.S. Transportation Secretary Ray LaHood, in a visit to the explosion site in April, called on pipeline operators to find areas of high risk and move quickly to repair and replace the pipes.

UGI said it was replacing an old pipe when the explosion happened on Thursday.

"We have an ongoing process where we replace cast iron and infrastructure based on certain criteria," said spokesman Dan Adamo. "This was one of those facilities that we were currently replacing through our standard procedures that we do when we replace facilities and upgrade them to plastic pipe."

UGI says it has contacted the Pennsylvania Public Utilities Commission about the blast, and the investigation into what happened is ongoing.

UGI said it is also doing leak surveys in the area to make sure everything is safe.