Speaking to CNBC in an interview on Wednesday, United CEO Oscar Munoz said that so far, the government shutdown has not had a “significant” impact on the airline. But as the shutdown continues on, that could change.

“Clearly the longer this goes, of course there’s going to be impact, and we do worry about that,” he said.

Now closing in on its fourth week, the shutdown’s impact is starting to be felt more heavily in certain industries, including airlines. On Tuesday, Delta said that it expects to take a $25 million revenue hit this month because of fewer government employees traveling on its network. Over time, if the shutdown continues, those revenue drops could widen.

The shutdown is showing no signs of coming to a close anytime soon. President Trump and Democrats are still at odds over border security funding and Trump has shown no willingness to relent in his stance and reopen the government.

Meanwhile, hundreds of thousands of government employees deemed “essential” have been forced to work without a paycheck. Others who have been deemed “non-essential” have been furloughed until the government reopens.

Looking ahead, Munoz said that he anticipates first-quarter revenue to range from flat to 3% higher compared to last year’s figures. He told CNBC that his range is wider now because of uncertainty surrounding the shutdown.