Delta Air Lines CEO Ed Bastian said his company has one message for United Technologies after it announced its merger with Raytheon: "Stay focused."

"UTC is a big partner of ours, a big supplier of ours," Bastian said in an interview with CNBC's Carl Quintanilla. "They've gone through some mergers, there's been some distractions along the way."

Bastian's comments come as some investors and activists speak out against the planned deal.

Raytheon and United Technologies on Sunday announced they would merge in an all-stock deal, creating a company with combined annual sales of $74 billion. When the merger is complete, it will be renamed Raytheon Technologies.

United Technologies owns aerospace manufacturer Pratt & Whitney, which produces engines for many of Delta's planes. Delta jets also have a number of products aboard them made by Collins Aerospace, another subsidiary of United Technologies.

If the deal goes through, shareholders of United Technologies will own 57% of the new company's stock, while Raytheon investors would own 43% on a diluted basis.

However, some activists are pressing United Technologies to rethink the merger.

Billionaire investor Bill Ackman sent a letter to United Technologies CEO Greg Hayes urging the company to rethink its deal with Raytheon, a source told CNBC. Ackman, whose hedge fund Pershing Square Capital Management owned 5.8 million United Technologies shares at the end of the first quarter, expressed his unease about the deal.