(Reuters) - Major U.S. companies including Ford, Apple and Pfizer have formed a lobbying group aimed at pushing back at some changes to the patent system members of Congress have proposed, saying these measures would hinder protection of valuable inventions.

The group is concerned about pending legislation aimed at fighting so-called patent assertion entities (PAEs), companies which produce nothing but instead buy up patents and then attempt to extract licensing fees or sue for infringement.

Called the Partnership for American Innovation, the group warned that steps to stop the PAEs could also hurt truly innovative companies.

Companies signing on to the effort so far are Apple Inc., DuPont, Ford Motor Co., General Electric, IBM Corp, Microsoft Corp and Pfizer Inc.

"There's a feeling that the negative rhetoric is leading to a very anti-patent environment," said David Kappos, director of the U.S. Patent and Trademark Office from 2009 to 2013, who advises the group. He is with the law firm Cravath, Swaine and Moore, LLP.

In particular, the group would oppose efforts to make software or biotechnology unpatentable.

Google, Cisco and other supporters of efforts to curb frivolous patent litigation from PAEs, often termed "patent trolls," supported a bill that easily passed the U.S. House of Representatives in December.

That bill encourages judges hearing patent cases to require the losing party to cover the winners' legal bills, a concept known as "loser pays." It also requires companies filing infringement lawsuits to detail which patent is infringed - something that does not regularly happen.

A separate Senate patent bill is scheduled to be marked up soon.

(Reporting by Diane Bartz, editing by Ros Krasny and David Gregorio)

WASHINGTON  Thu Apr 3, 2014 10:03am EDT