Hundreds of people living near a natural-gas drilling site in northwest Louisiana have been forced to evacuate their homes after gas seeped into their drinking water.

Authorities in Caddo Parish evacuated at least 135 homes just south of Shreveport on Monday and Tuesday after a well being drilled nearby began spewing gas into the air and tests showed gas in local drinking water. Those who left can’t return until Wednesday at the earliest, authorities said.

“We’re erring on the side of safety,” said Parish Commissioner Michael Thibodeaux, who represents the evacuated area.

Caddo Parish lies at the heart of the Haynesville Shale, a huge natural-gas field discovered in 2008. The field and others like it in Texas, Pennsylvania and other states have helped drive a boom in U.S. natural-gas production in recent years.

As drilling has spread to new and more heavily populated areas, however, some residents have become increasingly concerned about the possibility of air and water contamination. The industry says its practices are safe and argues that companies have drilled tens of thousands of wells in recent years with only a handful of incidents.

Problems in Caddo Parish began Sunday evening when a well being drilled by Exco Resources Inc., a Dallas-based gas producer, struck a pocket of gas much shallower than the company expected. Workers tried to control the well, but gas escaped into the air. Gas was also found in shallow freshwater aquifer that provides drinking water to many residents. Investigators will seek to confirm any link.

Exco immediately notified local authorities, who began evacuating residents early Monday morning. Subsequent tests found high levels of gas in dozens of local water wells, in some cases at levels that could lead to an explosion. “We didn’t want people to have [gas] build up in their house and all of a sudden they have an explosive situation,” said Otis Randle, regional director for the state Department of Environmental Quality, which conducted the tests.

Methane, the primary component of natural gas, can dissolve in water or can travel as a gas through the same porous rock that holds water tapped by drinking wells.

The evacuation was voluntary, but residents who stay behind can’t use their water. Exco is paying for evacuated residents’ hotel rooms. Mr. Randle said the water contamination hasn’t been definitively linked to Exco’s drilling operations, although it appears to be centered around the well site.

Exco Chief Operating Officer Harold Hickey said the firm is burning off any gas being released and will permanently shut the well. He said Exco has drilled about 50 Haynesville Shale wells without such an incident, and is cooperating with investigators.

“We really want to make sure we understand what the source of the natural gas is,” Mr. Hickey said.

Write to Ben Casselman at ben.casselman@wsj.com