Three of the largest U.S. airlines have inched up fares in what could mark the first industrywide hike since June.

Delta Air Lines Inc increased prices Monday on flights by up to $4 one-way, travel search website FareCompare said Tuesday, adding that Southwest Airlines Co has followed suit.

American Airlines Group Inc also raised U.S. domestic fares by $3 one-way to match its rivals, company spokesman Josh Freed told Reuters.

While U.S. airlines regularly adjust their fares, hiking prices for nearly all domestic flights is less common and an industry-wide match even less so. The moves come as the U.S. Justice Department investigates whether carriers have worked together illegally to keep fares high by signaling plans to limit flights.

Delta, Southwest, United Continental Holdings Inc and JetBlue Airways Corp did not immediately return requests for comment.

It was not immediately clear what prompted the price increases.

"We are confident in full industry-matching over the next several hours," JPMorgan analyst Jamie Baker said in a research note. "Just as Southwest often plays the spoiler role, competitors consistently follow Southwest's lead" when it raises prices.

Baker said the Southwest's fare increase was also $3 one-way.