U.S. regulators are putting together a possible antitrust challenge to Google Inc's planned $700 million acquisition of airline ticketing software company ITA Software, sources knowledgeable about the deal said on Thursday.

The proposed deal has sparked worries in the tech world that travel websites such as those of Expedia Inc, Orbitz Worldwide Inc, Kayak and TripAdvisor could be deprived of ITA's critical software.

Officials in the Justice Department's antitrust division have asked for data and documents about the merger and said they wanted the information fast, the sources said.

DOJ calls up and says 'We're under time pressure,' said one source.

People (at the Justice Department) had told us that they had canceled Christmas plans, a second person said.

My own indication is that I don't see any signs that the parties are going to resolve this amicably and the likelihood as we sit today is that it will go to court, the first source said. But that could change.

Bloomberg reported that Google had invoked a provision of federal law that forces the government to decide within 30 days whether to challenge the deal.

A Google representative did not return telephone calls seeking comment. Google is the world's No. 1 Internet search engine.

A Justice Department spokeswoman could not immediately be reached for comment.

Obviously they're thinking about bringing a lawsuit, said Bob Doyle of the law firm Doyle, Barlow and Mazard PLLC.

Doyle argued that it would be difficult to fight the merger in court because Google -- despite its heft in the tech world -- is not in the same business as ITA. We think it's a bad case. We don't think the government has a good case, he said.

Google has said that it would honor contracts that ITA has with the travel sites but has declined to go further.

Google argues that since it does not compete against ITA Software, the deal would not change existing market share in the online travel industry and would result in better search results for consumers.

(Reporting by Alexei Oreskovic and Diane Bartz. Editing by Robert MacMillan)